Refactor performance memory API implementation

Summary:
This diff refactors performance memory API and the RN Tester example.

- The returned value from C++ is number, so no need to cast
- Reuse `MemoryInfo` in RNTester example

Changelog:
[General][Internal] - Code refactor for performance memory API implementation

Reviewed By: rubennorte

Differential Revision: D43523878

fbshipit-source-id: 37d1f6a829a8eac45f7e3791ad36be0c199c6041