notificationBroadcasts shouldn't be a StateFlow

As a stateflow, it doesn't function correctly because we actually do expect duplicate emissions to be meaningful, so we need to keep them. I think it also doesn't need to be a SharedFlow, since I'm not sure the flowOn does anything in that case...

Bug: 346519570
Bug: 347707024
Change-Id: I64da785532d748a27173fda35bcb1f8c21cc42e6
Test: manually logged emissions to test
Flag: com.android.settingslib.flags.volume_panel_broadcast_fix