Errores de Proguard al generar APK firmado

Tengo minifyEnabled establecido en true en mi archivo build.gradle. Tengo muchos errores al generar un APK firmado para mi aplicación:

Warning:com.dropbox.core.http.OkHttp3Requestor$AsyncCallback: can't find superclass or interface okhttp3.Callback Warning:com.dropbox.core.http.OkHttp3Requestor$PipedRequestBody: can't find superclass or interface okhttp3.RequestBody Warning:com.dropbox.core.http.OkHttpRequestor$AsyncCallback: can't find superclass or interface com.squareup.okhttp.Callback Warning:com.dropbox.core.http.OkHttpRequestor$PipedRequestBody: can't find superclass or interface com.squareup.okhttp.RequestBody Warning:com.dropbox.core.DbxStandardSessionStore: can't find referenced class javax.servlet.http.HttpSession Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.URLFetchServiceFactory Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.HTTPMethod Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.URLFetchService Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.HTTPMethod Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.HTTPRequest Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.HTTPHeader Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.HTTPRequest Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.FetchOptions$Builder Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.FetchOptions Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.HTTPResponse Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.HTTPHeader Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.HTTPResponse Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.FetchOptions Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.URLFetchService Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.HTTPRequest Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.HTTPHeader Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.URLFetchServiceFactory Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.HTTPMethod Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.URLFetchService Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.FetchOptions$Builder Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.FetchOptions Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.HTTPResponse Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.URLFetchService Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.FetchOptions Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.URLFetchService Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.FetchOptions Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.URLFetchService Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.FetchOptions Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.URLFetchService Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.HTTPRequest Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.HTTPResponse Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.HTTPRequest Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.HTTPMethod Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.HTTPRequest Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.HTTPMethod Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.HTTPRequest Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.HTTPMethod Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.HTTPRequest Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.FetchOptions Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.HTTPResponse Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.HTTPHeader Warning:com.dropbox.core.http.GoogleAppEngineRequestor: can't find referenced class com.google.appengine.api.urlfetch.HTTPResponse Warning:com.dropbox.core.http.GoogleAppEngineRequestor$FetchServiceUploader: can't find referenced class com.google.appengine.api.urlfetch.HTTPRequest Warning:com.dropbox.core.http.GoogleAppEngineRequestor$FetchServiceUploader: can't find referenced class com.google.appengine.api.urlfetch.URLFetchService Warning:com.dropbox.core.http.GoogleAppEngineRequestor$FetchServiceUploader: can't find referenced class com.google.appengine.api.urlfetch.HTTPRequest Warning:com.dropbox.core.http.GoogleAppEngineRequestor$FetchServiceUploader: can't find referenced class com.google.appengine.api.urlfetch.URLFetchService Warning:com.dropbox.core.http.GoogleAppEngineRequestor$FetchServiceUploader: can't find referenced class com.google.appengine.api.urlfetch.HTTPRequest Warning:com.dropbox.core.http.GoogleAppEngineRequestor$FetchServiceUploader: can't find referenced class com.google.appengine.api.urlfetch.URLFetchService Warning:com.dropbox.core.http.GoogleAppEngineRequestor$FetchServiceUploader: can't find referenced class com.google.appengine.api.urlfetch.HTTPRequest Warning:com.dropbox.core.http.GoogleAppEngineRequestor$FetchServiceUploader: can't find referenced class com.google.appengine.api.urlfetch.URLFetchService Warning:com.dropbox.core.http.GoogleAppEngineRequestor$FetchServiceUploader: can't find referenced class com.google.appengine.api.urlfetch.HTTPRequest Warning:com.dropbox.core.http.GoogleAppEngineRequestor$FetchServiceUploader: can't find referenced class com.google.appengine.api.urlfetch.HTTPResponse Warning:com.dropbox.core.http.OkHttp3Requestor: can't find referenced class okhttp3.OkHttpClient$Builder Warning:com.dropbox.core.http.OkHttp3Requestor: can't find referenced class okhttp3.OkHttpClient Warning:com.dropbox.core.http.OkHttp3Requestor: can't find referenced class okhttp3.Dispatcher Warning:com.dropbox.core.http.OkHttp3Requestor: can't find referenced class okhttp3.Request$Builder Warning:com.dropbox.core.http.OkHttp3Requestor: can't find referenced class okhttp3.OkHttpClient Warning:com.dropbox.core.http.OkHttp3Requestor: can't find referenced class okhttp3.Call Warning:com.dropbox.core.http.OkHttp3Requestor: can't find referenced class okhttp3.Response Warning:com.dropbox.core.http.OkHttp3Requestor: can't find referenced class okhttp3.ResponseBody Warning:com.dropbox.core.http.OkHttp3Requestor: can't find referenced class okhttp3.Request$Builder Warning:com.dropbox.core.http.OkHttp3Requestor: can't find referenced class okhttp3.Headers Warning:com.dropbox.core.http.OkHttp3Requestor: can't find referenced class okhttp3.OkHttpClient Warning:com.dropbox.core.http.OkHttp3Requestor: can't find referenced class okhttp3.Dispatcher Warning:com.dropbox.core.http.OkHttp3Requestor: can't find referenced class okhttp3.Request Warning:com.dropbox.core.http.OkHttp3Requestor: can't find referenced class okhttp3.Call Warning:com.dropbox.core.http.OkHttp3Requestor: can't find referenced class okhttp3.Response Warning:com.dropbox.core.http.OkHttp3Requestor: can't find referenced class okhttp3.ResponseBody Warning:com.dropbox.core.http.OkHttp3Requestor: can't find referenced class okhttp3.Headers Warning:com.dropbox.core.http.OkHttp3Requestor: can't find referenced class okhttp3.OkHttpClient Warning:com.dropbox.core.http.OkHttp3Requestor: can't find referenced class okhttp3.Request$Builder Warning:com.dropbox.core.http.OkHttp3Requestor: can't find referenced class okhttp3.Response Warning:com.dropbox.core.http.OkHttp3Requestor: can't find referenced class okhttp3.Request$Builder Warning:com.dropbox.core.http.OkHttp3Requestor: can't find referenced class okhttp3.Response Warning:com.dropbox.core.http.OkHttp3Requestor: can't find referenced class okhttp3.Request$Builder Warning:com.dropbox.core.http.OkHttp3Requestor: can't find referenced class okhttp3.Headers Warning:com.dropbox.core.http.OkHttp3Requestor: can't find referenced class okhttp3.OkHttpClient Warning:com.dropbox.core.http.OkHttp3Requestor: can't find referenced class okhttp3.Headers Warning:com.dropbox.core.http.OkHttp3Requestor$AsyncCallback: can't find referenced class okhttp3.Callback Warning:com.dropbox.core.http.OkHttp3Requestor$AsyncCallback: can't find referenced class okhttp3.Response Warning:com.dropbox.core.http.OkHttp3Requestor$AsyncCallback: can't find referenced class okhttp3.Call Warning:com.dropbox.core.http.OkHttp3Requestor$AsyncCallback: can't find referenced class okhttp3.Response Warning:com.dropbox.core.http.OkHttp3Requestor$AsyncCallback: can't find referenced class okhttp3.Call Warning:com.dropbox.core.http.OkHttp3Requestor$AsyncCallback: can't find referenced class okhttp3.Response Warning:com.dropbox.core.http.OkHttp3Requestor$BufferedUploader: can't find referenced class okhttp3.Request$Builder Warning:com.dropbox.core.http.OkHttp3Requestor$BufferedUploader: can't find referenced class okhttp3.OkHttpClient Warning:com.dropbox.core.http.OkHttp3Requestor$BufferedUploader: can't find referenced class okhttp3.Call Warning:com.dropbox.core.http.OkHttp3Requestor$BufferedUploader: can't find referenced class okhttp3.Request$Builder Warning:com.dropbox.core.http.OkHttp3Requestor$BufferedUploader: can't find referenced class okhttp3.RequestBody Warning:com.dropbox.core.http.OkHttp3Requestor$BufferedUploader: can't find referenced class okhttp3.Call Warning:com.dropbox.core.http.OkHttp3Requestor$BufferedUploader: can't find referenced class okhttp3.Response Warning:com.dropbox.core.http.OkHttp3Requestor$BufferedUploader: can't find referenced class okhttp3.ResponseBody Warning:com.dropbox.core.http.OkHttp3Requestor$BufferedUploader: can't find referenced class okhttp3.Request$Builder Warning:com.dropbox.core.http.OkHttp3Requestor$BufferedUploader: can't find referenced class okhttp3.Response Warning:com.dropbox.core.http.OkHttp3Requestor$BufferedUploader: can't find referenced class okhttp3.OkHttpClient Warning:com.dropbox.core.http.OkHttp3Requestor$BufferedUploader: can't find referenced class okhttp3.Call Warning:com.dropbox.core.http.OkHttp3Requestor$BufferedUploader: can't find referenced class okhttp3.RequestBody Warning:com.dropbox.core.http.OkHttp3Requestor$BufferedUploader: can't find referenced class okhttp3.Response Warning:com.dropbox.core.http.OkHttp3Requestor$BufferedUploader: can't find referenced class okhttp3.ResponseBody Warning:com.dropbox.core.http.OkHttp3Requestor$BufferedUploader: can't find referenced class okhttp3.Request Warning:com.dropbox.core.http.OkHttp3Requestor$BufferedUploader: can't find referenced class okhttp3.Request$Builder Warning:com.dropbox.core.http.OkHttp3Requestor$BufferedUploader: can't find referenced class okhttp3.RequestBody Warning:com.dropbox.core.http.OkHttp3Requestor$BufferedUploader: can't find referenced class okhttp3.Call Warning:com.dropbox.core.http.OkHttp3Requestor$BufferedUploader: can't find referenced class okhttp3.Request$Builder Warning:com.dropbox.core.http.OkHttp3Requestor$BufferedUploader: can't find referenced class okhttp3.RequestBody Warning:com.dropbox.core.http.OkHttp3Requestor$BufferedUploader: can't find referenced class okhttp3.Response Warning:com.dropbox.core.http.OkHttp3Requestor$PipedRequestBody: can't find referenced class okhttp3.RequestBody Warning:com.dropbox.core.http.OkHttp3Requestor$PipedRequestBody: can't find referenced class okhttp3.MediaType Warning:com.dropbox.core.http.OkHttp3Requestor$PipedRequestBody: can't find referenced class okio.BufferedSink Warning:com.dropbox.core.http.OkHttpRequestor: can't find referenced class com.squareup.okhttp.OkHttpClient Warning:com.dropbox.core.http.OkHttpRequestor: can't find referenced class com.squareup.okhttp.Dispatcher Warning:com.dropbox.core.http.OkHttpRequestor: can't find referenced class com.squareup.okhttp.OkHttpClient Warning:com.dropbox.core.http.OkHttpRequestor: can't find referenced class com.squareup.okhttp.Request$Builder Warning:com.dropbox.core.http.OkHttpRequestor: can't find referenced class com.squareup.okhttp.OkHttpClient Warning:com.dropbox.core.http.OkHttpRequestor: can't find referenced class com.squareup.okhttp.Call Warning:com.dropbox.core.http.OkHttpRequestor: can't find referenced class com.squareup.okhttp.Response Warning:com.dropbox.core.http.OkHttpRequestor: can't find referenced class com.squareup.okhttp.ResponseBody Warning:com.dropbox.core.http.OkHttpRequestor: can't find referenced class com.squareup.okhttp.Request$Builder Warning:com.dropbox.core.http.OkHttpRequestor: can't find referenced class com.squareup.okhttp.Headers Warning:com.dropbox.core.http.OkHttpRequestor: can't find referenced class com.squareup.okhttp.OkHttpClient Warning:com.dropbox.core.http.OkHttpRequestor: can't find referenced class com.squareup.okhttp.Dispatcher Warning:com.dropbox.core.http.OkHttpRequestor: can't find referenced class com.squareup.okhttp.Request Warning:com.dropbox.core.http.OkHttpRequestor: can't find referenced class com.squareup.okhttp.Call Warning:com.dropbox.core.http.OkHttpRequestor: can't find referenced class com.squareup.okhttp.Response Warning:com.dropbox.core.http.OkHttpRequestor: can't find referenced class com.squareup.okhttp.ResponseBody Warning:com.dropbox.core.http.OkHttpRequestor: can't find referenced class com.squareup.okhttp.Headers Warning:com.dropbox.core.http.OkHttpRequestor: can't find referenced class com.squareup.okhttp.OkHttpClient Warning:com.dropbox.core.http.OkHttpRequestor: can't find referenced class com.squareup.okhttp.Request$Builder Warning:com.dropbox.core.http.OkHttpRequestor: can't find referenced class com.squareup.okhttp.Response Warning:com.dropbox.core.http.OkHttpRequestor: can't find referenced class com.squareup.okhttp.Request$Builder Warning:com.dropbox.core.http.OkHttpRequestor: can't find referenced class com.squareup.okhttp.Response Warning:com.dropbox.core.http.OkHttpRequestor: can't find referenced class com.squareup.okhttp.Request$Builder Warning:com.dropbox.core.http.OkHttpRequestor: can't find referenced class com.squareup.okhttp.Headers Warning:com.dropbox.core.http.OkHttpRequestor: can't find referenced class com.squareup.okhttp.OkHttpClient Warning:com.dropbox.core.http.OkHttpRequestor: can't find referenced class com.squareup.okhttp.Headers Warning:com.dropbox.core.http.OkHttpRequestor$AsyncCallback: can't find referenced class com.squareup.okhttp.Callback Warning:com.dropbox.core.http.OkHttpRequestor$AsyncCallback: can't find referenced class com.squareup.okhttp.Response Warning:com.dropbox.core.http.OkHttpRequestor$AsyncCallback: can't find referenced class com.squareup.okhttp.Request Warning:com.dropbox.core.http.OkHttpRequestor$AsyncCallback: can't find referenced class com.squareup.okhttp.Response Warning:com.dropbox.core.http.OkHttpRequestor$BufferedUploader: can't find referenced class com.squareup.okhttp.Request$Builder Warning:com.dropbox.core.http.OkHttpRequestor$BufferedUploader: can't find referenced class com.squareup.okhttp.OkHttpClient Warning:com.dropbox.core.http.OkHttpRequestor$BufferedUploader: can't find referenced class com.squareup.okhttp.Call Warning:com.dropbox.core.http.OkHttpRequestor$BufferedUploader: can't find referenced class com.squareup.okhttp.Request$Builder Warning:com.dropbox.core.http.OkHttpRequestor$BufferedUploader: can't find referenced class com.squareup.okhttp.RequestBody Warning:com.dropbox.core.http.OkHttpRequestor$BufferedUploader: can't find referenced class com.squareup.okhttp.Call Warning:com.dropbox.core.http.OkHttpRequestor$BufferedUploader: can't find referenced class com.squareup.okhttp.Response Warning:com.dropbox.core.http.OkHttpRequestor$BufferedUploader: can't find referenced class com.squareup.okhttp.ResponseBody Warning:com.dropbox.core.http.OkHttpRequestor$BufferedUploader: can't find referenced class com.squareup.okhttp.Request$Builder Warning:com.dropbox.core.http.OkHttpRequestor$BufferedUploader: can't find referenced class com.squareup.okhttp.Response Warning:com.dropbox.core.http.OkHttpRequestor$BufferedUploader: can't find referenced class com.squareup.okhttp.OkHttpClient Warning:com.dropbox.core.http.OkHttpRequestor$BufferedUploader: can't find referenced class com.squareup.okhttp.Call Warning:com.dropbox.core.http.OkHttpRequestor$BufferedUploader: can't find referenced class com.squareup.okhttp.RequestBody Warning:com.dropbox.core.http.OkHttpRequestor$BufferedUploader: can't find referenced class com.squareup.okhttp.Response Warning:com.dropbox.core.http.OkHttpRequestor$BufferedUploader: can't find referenced class com.squareup.okhttp.ResponseBody Warning:com.dropbox.core.http.OkHttpRequestor$BufferedUploader: can't find referenced class com.squareup.okhttp.Request Warning:com.dropbox.core.http.OkHttpRequestor$BufferedUploader: can't find referenced class com.squareup.okhttp.Request$Builder Warning:com.dropbox.core.http.OkHttpRequestor$BufferedUploader: can't find referenced class com.squareup.okhttp.RequestBody Warning:com.dropbox.core.http.OkHttpRequestor$BufferedUploader: can't find referenced class com.squareup.okhttp.Call Warning:com.dropbox.core.http.OkHttpRequestor$BufferedUploader: can't find referenced class com.squareup.okhttp.Request$Builder Warning:com.dropbox.core.http.OkHttpRequestor$BufferedUploader: can't find referenced class com.squareup.okhttp.RequestBody Warning:com.dropbox.core.http.OkHttpRequestor$BufferedUploader: can't find referenced class com.squareup.okhttp.Response Warning:com.dropbox.core.http.OkHttpRequestor$PipedRequestBody: can't find referenced class com.squareup.okhttp.RequestBody Warning:com.dropbox.core.http.OkHttpRequestor$PipedRequestBody: can't find referenced class com.squareup.okhttp.MediaType Warning:com.dropbox.core.http.OkHttpRequestor$PipedRequestBody: can't find referenced class okio.BufferedSink Warning:com.dropbox.core.http.OkHttpUtil$PipedStream: can't find referenced class okio.Okio Warning:com.dropbox.core.http.OkHttpUtil$PipedStream: can't find referenced class okio.BufferedSink Warning:com.dropbox.core.http.OkHttpUtil$PipedStream: can't find referenced class okio.Okio Warning:com.dropbox.core.http.OkHttpUtil$PipedStream: can't find referenced class okio.BufferedSink Warning:com.google.common.cache.Striped64: can't find referenced class sun.misc.Unsafe Warning:com.google.common.cache.Striped64$1: can't find referenced class sun.misc.Unsafe Warning:com.google.common.cache.Striped64$Cell: can't find referenced class sun.misc.Unsafe Warning:com.google.common.primitives.UnsignedBytes$LexicographicalComparatorHolder$UnsafeComparator: can't find referenced class sun.misc.Unsafe Warning:com.google.common.primitives.UnsignedBytes$LexicographicalComparatorHolder$UnsafeComparator$1: can't find referenced class sun.misc.Unsafe Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.OkHttpClient Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.Cache Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.OkHttpClient Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.CacheControl Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.CacheControl$Builder Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.Request$Builder Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.OkHttpClient Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.Call Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.Response Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.ResponseBody Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.Response Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.ResponseBody Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.OkHttpClient Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.Cache Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.CacheControl Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.CacheControl$Builder Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.Request$Builder Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.Response Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.Cache Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.CacheControl Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.Request Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.Call Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.Response Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.ResponseBody Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.OkHttpClient Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.CacheControl$Builder Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.CacheControl Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.Request$Builder Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.Response Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.ResponseBody Warning:com.squareup.picasso.OkHttpDownloader: can't find referenced class com.squareup.okhttp.Cache Warning:there were 355 unresolved references to classes or interfaces. Warning:Exception while processing task java.io.IOException: Please correct the above warnings first. Error:Execution failed for task ':app:transformClassesAndResourcesWithProguardForRelease'. > java.io.IOException: Please correct the above warnings first. 

Así que añadí estas líneas en el archivo progard-rules.pro:

 -keep com.dropbox.** {*;} -keep com.google.common.** {*;} -keep com.square.picasso.** {*;} 

Pero entonces, consigo este error:

 Warning:Exception while processing task java.io.IOException: proguard.ParseException: Unexpected keyword 'com.dropbox.**' in line 18 of file '/home/AndroidProjects/Poolex/app/proguard-rules.pro' Error:Execution failed for task ':app:transformClassesAndResourcesWithProguardForRelease'. > java.io.IOException: proguard.ParseException: Unexpected keyword 'com.dropbox.**' in line 18 of file '/home/AndroidProjects/Poolex/app/proguard-rules.pro' 

¿Cómo puedo solucionar esto?

Si lo desea, puede ignorar todos los errores de proguard y probar si la aplicación funciona. yo uso esto
-dontwarn **
Para ignorar todos los errores. Si no funciona, pruebe con:
-keep **
Y si quieres hacerlo con sólo uno, usa como
-keep class com.dropbox.**
o
-ignore com.dropbox.**

Prueba esto:

 -keep class com.dropbox.** {*;} -keep class com.google.common.** {*;} -keep class com.square.picasso.** {*;} 
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