Recalculate max notifications on lockscreen

NSSL has a listener that will be invoked when a notification changes,
and can be used to recalculate the number of notifications to show on
lockscreen. Use this signal to emit an event for the view model to
use.

Fixes: 296606746
Test: atest SharedNotificationContainerViewModelTest
Test: use Notify apk to add delayed notifications, ensure it stays to
the bounds
Flag: LEGACY MIGRATE_NSSL DISABLED

Change-Id: I7b59b56d167d2dec12761bda80fbea27f74bf239