Codegen: Add JNI C++ generator for TurboModule subclasses

Summary:
This adds the C++ classes for Android, just like checked in here: https://github.com/facebook/react-native/tree/master/ReactAndroid/src/main/java/com/facebook/fbreact/specs/jni

The whitespace/formatting is slightly different but the functionality should be preserved.

Changelog: [Internal]

Reviewed By: mdvacca

Differential Revision: D23489368

fbshipit-source-id: 60c112a48b6de2d2eb19f7d1bcc894048334610f