Give FirebaseLoginBaseActivity a `getAuth()` method, make `onFirebaseLoggedIn()` and `onFirebaseLoggedOut()` non-abstract

[puf]

The Firebase SDK allows you to call getAuth() to determine the currently authenticated user. I find myself now often overriding onFirebaseLoggedIn() and onFirebaseLoggedOut() solely to set or clear an mAuthData field.

Given how common this use-case is, the base class should contain auth data and expose it with a getAuth() method. This also means that onFirebaseLoggedIn() and onFirebaseLoggedOut() should become non-abstract. They are still useful, but many subclasses may not need them anymore.