Ensure notification listener is connected before getting notifications

There was a potential race condition where we would try to do a full
refresh of notifications before the listener was connected. Now we
skip the full refresh if we aren't connected, which is fine because
we also do a full refresh in onListenerConnected() anyway.

Bug: 62107069
Change-Id: I2fe4f49eb8a840230d32034be11fe7765a366e75