Summary:
@public
Moves all Java classes used from C++ to their own header + implementation file. In `YGJNI.cpp`, we only call methods on these objects deriving from `jni::JavaClass`.
This is only a bit of cleanup, no functional change.
Reviewed By: SidharthGuglani
Differential Revision: D14261043
fbshipit-source-id: 2db1d81cb7f56a4cdc24b182b2166e1d7e24ba3c
