Summary: Sometimes the main component name is not known at activity construction time and depends on e.g. reading shared preferences. To support this use case, make `(Fragment)ReactActivity#getMainComponentName()` nullable and return `null` by default. In this case, the app will not be loaded in `onCreate` by default and the user has to call `loadApp` manually once the component name is known.
Reviewed By: andreicoman11
Differential Revision: D3722517
fbshipit-source-id: 062eed158798606e4160f1c142b23fd98ca618c8
Summary: Add FragmentActivity-based ReactFragmentActivity to support apps using the v4 support lib. Add delegate class to share implementation details between the new class and ReactActivity.
Reviewed By: astreet
Differential Revision: D3655428
fbshipit-source-id: d3ff916538e13b6f0d594bbb91555e322645e954
