react-native-firebase

- [dependencies][android] remove all Firebase SDKs in favour of Firebase BoM
- [android] upgrade to Android X
- [android] upgrade gradle wrapper to v5.4.1
- [android][ios][tests] remove manual packages & enable auto-linking
- [tests][internal] upgrade tests project to RN 60
- [ios] temporarily remove framework support in pods - broken in RN 60 - see https://github.com/facebook/react-native/issues/25349
- [linting] switch to use rn community linting rules