Update the bell button's image depending on available notifications.
- This task concerns the Bell button visible when logged in and on the Explore feed or when logged in and on the root Settings view if the Explore feed is turned off
- Show in indicated notification available state as requested by app
- Show in un-indicated plain state as requested by app
 The UI needs API on the Notifications Data Controller to interact with here
 What is the mechanism for triggering a state change (Explore/Settings root view appearance, notification, etc.)?