Summary:
Original Android's refreshControl in ScrollView is tightly coupled with AndroidSwipeRefreshLayout. If someone use `ref=` for RefreshControl in ScrollView, it does nothing since RefreshControl in Android return null.
This change allows customized RefreshControl especially for `ref=` as well as making ScrollView's code clearer.
Closes https://github.com/facebook/react-native/pull/5623
Reviewed By: svcscm
Differential Revision: D2890072
Pulled By: nicklockwood
fb-gh-sync-id: a8fc7746bcc050a6e46fedf3583979f4cb9021b6