Use new JavaScriptCore from npm (#22231)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/22231

- Use clang instead of the deprecated gcc
- Use libc++ instead of the deprecated gnustl
- Updated gradle and android plugin version
- Fixed missing arch in local-cli template
- `clean` task should now always succeed
- `clean` task deletes build artifacts
- No need to specify buildToolsVersion. It's derived.
- Elvis operator for more readable code
Pull Request resolved: https://github.com/facebook/react-native/pull/22263

Reviewed By: hramos

Differential Revision: D13004499

Pulled By: DanielZlotin

fbshipit-source-id: da54bb744cedb4c6f3bda590f8c25d0ad64086ef