Push Notifications

[abhijeetviswa]

This patchset introduces support for push notifications. Moodle doesn't support notifications for new modules and course sections. To add support for these, a parallel Moodle plugin is being developed at abhijeetviswa/moodle-local_extendednotifs.

[abhijeetviswa]

Checklist before merging (including work left, as well as cleanup):

• [x] Add Firebase notification icon.
• [x] Remove the TokenActivity launcher.
• [ ] Ensure invalid Moodle tokens are intercepted (possibly using this) and user is logged out. Could be in a separate PR.
• [x] Firebase tokens need to be committed to local storage. MyFirebaseMessagingService#newToken() calls can then check against this local store and correspondingly send a request to the Moodle instance.
• [x] Remove the test token added in TokenActivity#onCreated().
• [ ] Handle Firebase messages when app is in the foreground.
• [x] Remove the existing notification background worker.

[abhijeetviswa]

The remaining two unmarked items aren't critical to the functioning of the app and can be added in later one. LGTM first.