Stop subclassing RNFirebaseMessagingService as its functionality is not used anymore

Summary: This differential removes RNFirebaseMessagingService from native Android codebase as its functionality is not used anymore. FirebaseMessagingService is now subclassed.

Test Plan: Build the app. All tests plans of differentials int ths stack under this one.

Reviewers: tomek, bartek

Reviewed By: tomek, bartek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6275