Move CommNotificationsHandler to newly created notifications package

Summary: This differential creates specialized package for code intended to deal with notifications and places CommNotificationsHandler there. Future differentials will add more files to this package

Test Plan: This differential brings no functional change. Build the app and ensure that notification displaying (foreground and background) as well as rescinding works the same.

Reviewers: tomek, atul, ashoat

Reviewed By: tomek, ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D6226