Fix an ANR caused by the dream overlay status bar.

Specifically, this ANR happened when attempting to compute the
notification count in order to show a notification icon. The new
strategy reacts to notification additions/subtractions rather than
attempting to fetch all notifications every time the notifications
change.

Test: atest DreamOverlayStatusBarViewControllerTest,
DreamOverlayNotificationCountProviderTest
Bug: 225967369

Change-Id: Ied9453e8788448f957e5b70c7dae08022a993cc1