Android Embedding PR29: Improve FlutterFragment construction API + engine config API.

[matthew-carroll]

Android Embedding PR29: Improve FlutterFragment construction API + engine config API.

I accidentally removed the ability to override a FlutterFragment method for engine customization in a previous PR. I bring that ability back with this PR.

@Override
@Nullable
protected FlutterEngine createFlutterEngine(@NonNull Context context) {
  return MyEngineCache.cachedEngine;
} 

I also revamped the Builder within FlutterFragment such that FlutterFragment.Builder can be used directly for FlutterFragment subclasses without itself being subclassed, so long as the subclass does not need to introduce any new properties:

MyFlutterFragment f = new FlutterFragment.Builder(MyFlutterFragment.class)
  .someProperty(...)
  .build<MyFlutterFragment>();

If the FlutterFragment subclass does need to add more creation properties then the Builder can be subclassed as before. Then it can be used like:

MyFlutterFragment f = new MyFlutterFragment.MyBuilder()
  .someProperty(...)
  .build<MyFlutterFragment>();

If a typical FlutterFragment is desired, then the Builder can be used as it previously was:

FlutterFragment f = new FlutterFragment.Builder()
  .someProperty(...)
  .build();

[mjohnsullivan]

API changes LGTM