Reduce the amount of C++ code in user space for RN-Tester

Summary:
This diff reduces the amount of C++ code in user space by:
- Moving all the C++ implementation of the .defaults package inside a `react_newarchdefaults` shared library
- Exposing only the entrypoint logic inside the RN-Tester's `OnLoad.cpp` file.

Changelog:
[Android] [Changed] - Reduce the amount of C++ code in user space for New Architecture

Reviewed By: cipolleschi

Differential Revision: D39381820

fbshipit-source-id: 9c4b5596b67b5a7ee58824319c80e325348ed06c