Refactor android notifications native and JavaScript code so that JS can register for events using native constants

Summary: This differential refactors android notifications code so that JavaScript can register for native notifications events using event names exported as constants from native code instead of hardcoded strings.

Test Plan: Build the app. Ensure notifications functionalities are working correctly

Reviewers: tomek, bartek

Reviewed By: bartek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D8113