Optimize order of overridable methods in FirebaseLoginBaseActivity

[puf]

The first method you typically override first when you subclass FirebaseLoginBaseActivity is getRef(). That's why we show it first in the README.

But when you tell Android Studio to implement the abstract methods, it adds them to the subclass in the order they are defined in the base class. So getRef() should be before all the other abstract methods in FirebaseLoginBaseActivity.