NoClassDefFoundErrors logcat spam for optional providers

[wojtek-kalicinski]

• Android device: Pixel 2
• Android OS version: P
• Google Play Services version:
• Firebase/Play Services SDK version:
• FirebaseUI version: 3.2.2

Step 3: Describe the problem:

ART verification spams the logcat with NoClassDefFoundErrors, because it needs to resolve superclasses of TwitterProvider and FacebookProvider, which don't exist if you're not using these SDKs.

Instead of extending/implementing Callbacks from the SDKs directly, these classes should have inner classes for callbacks, that will not be eagerly verified.

Steps to reproduce:

1. Don't include Facebook and Twitter SDKs
2. Try to sign in using Firebase Auth

Observed Results:

NoClassDefFoundErrors in logcat

Expected Results:

No errors

[samtstern]

@SUPERCILEX integrated your suggestions into #1189

[samtstern]

This issue has been fixed and released in 3.3.0.