Summary:The `onRequestClose` prop can be used to handle dismissing the modal by back button. It's pretty easy to miss the `onRequestClose` prop on Android. So making it a required prop makes sure that it generates a warning when not supplied.
Context #6612
Closes https://github.com/facebook/react-native/pull/6667
Differential Revision: D3102054
Pulled By: javache
fb-gh-sync-id: 878240606285d231b5592a438918e441765bfe5f
fbshipit-source-id: 878240606285d231b5592a438918e441765bfe5f