Unify listener interfaces in NotificationListener

Now the NotificationListener is agnostic as to whether it's firing
events to the NEM, the NotifCollection, or both.

Should allow us to switch between the NEM and NotifCollection in the
future.

Also inlines the old feature flag ENABLE_CHILD_NOTIFICATIONS.

Bug: 112656837
Test: atest
Test: manual
Change-Id: Ic6a49c08db7a06c021f36c7f15c2dbd22ef27eaa