Connect volume keys to RSSI update on AttendeeListFragment

@spaikmos This PR has some quick changes I made to hook up the volume key events to updating the attendee list. Unfortunately, due to the way in which you're clearing an re-populating the list every time, the whole list is getting refreshed along with the RSSI status on RonnieL's row. I think you can add a method in your EventUserArrayAdapter class to find all users who are at the event, get a reference to the RSSI view object, and redraw the RSSI view. You can then call that method in the onKeyPressed() interface implementation in AttendeeListFragment to update just the RSSI view when the volume keys are pressed. I thought this would give you a good start though. Let me know if you have any questions!