[android] Didn't find class "com.google.firebase.provider.FirebaseInitProvider"?

I had the same problem in my (YouTube player project)... and the following solved the problem for me:

  1. Add this code into your build.gradle (module: app) inside defaultConfing:

    defaultConfig {
        ....
        ....
        multiDexEnabled = true
    }
    
  2. Add this code into your build.gradle (module: app) inside dependencies:

    dependencies {
        compile 'com.android.support:multidex:1.0.1'
        .....
        .....
    }
    
  3. Open AndroidManifest.xml and within application:

    <application
        android:name="android.support.multidex.MultiDexApplication"
        .....
        .....
    </application>
    

    or if you have your App class, extend it from MultiDexApplication like:

    public class MyApp extends MultiDexApplication {
    .....
    

And finally, I think you should have Android Support Repository downloaded, in the Extras in SDK Manager.