Refactor CommIOSNotifications native and JavaScript code so that JavaScript registers to events using getConstants object instead of hardcoded strings

Summary: This diferential refactors code so that JavaScript can access notifications related event names directly from getConstants property of CommIOSNotifications

Test Plan: Build the app. Test that notifications functionalities work

Reviewers: tomek, bartek

Reviewed By: bartek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D8106