Export Background Fetch result constants from CommIOSNotifications to JavaScript

Summary: This differential implements method in CommIOSNotifications that allows JavaScript to use native string constants that describe background fetch result, without direct hardcoding

Test Plan: Log CommIOSNotifications.getConstants() somwhere in JS and examine its content

Reviewers: tomek, atul

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D6139