Apply react.gradle from node_modules/react-native

Summary:The goal is to minimize the number of files we need to bootstrap. This allows us to make the upgrade process smoother for everyone.

If someone needs to customize the file, we already provide some config options. The ability to copy the file and modify it is always there for those few who need it.

**Test plan**

Generate a new project with the updated template.  The app should build and run fine both in debug and production mode.

Related #6292
Closes https://github.com/facebook/react-native/pull/6610

Differential Revision: D3109099

Pulled By: foghina

fb-gh-sync-id: 13fc89e60daed30bf6349e532a140c1b6f8f053a
fbshipit-source-id: 13fc89e60daed30bf6349e532a140c1b6f8f053a