Deprecate react.gradle
Deprecate react.gradle

Summary:
With RNGP we can now deprecate react.gradle
I'll create a page on the website to describe what is the migration strategy here,
but users should be fine by just replacing `apply from react.gradle` with
`apply plugin: com.facebook.react`.

Changelog:
[Android] [Removed] - Deprecate react.gradle

Reviewed By: cipolleschi

Differential Revision: D40675546

fbshipit-source-id: a50348791d669df1ed9e0470a6664e291c1a4584

facebook/react-native,facebook/react-native,facebook/react-native,facebook/react-native,facebook/react-native,facebook/react-native,facebook/react-native,facebook/react-native,facebook/react-native