RNGP - Various improvements needed for 3rd party libs (#35496)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/35496

This commit includes a series of fixes needed for better integration with libraries for 0.71:
- I've added an `android/README.md` file as some libraries were failing the build if the folder was missing
- RNGP now applies dep substitution on app and all the libraries project
- RNGP now adds repositories on app and all the libraries project
- I've removed the maven local repo to the `/android` folder as now is empty
- I've fixed the path for the JSC repo for Windows users
- I've added a bit of backward compat by re-adding an empty `project.react.ext` block that libraries might read from.
- I've removed `codegenDir` from the `GenerateCodegenArtifactsTask` which was unused.

Changelog:
[Internal] [Changed] - RNGP - Various improvements needed for 3rd party libs

Reviewed By: cipolleschi

Differential Revision: D41549489

fbshipit-source-id: 2252da0180ac24fd3fe5a55300527da6781f0f8c