Fix race condition in starting AutoDJ

* Better support for multiple decks: switch from loadTrack signal (which has an
  implicit target) to loadTrackToPlayer so we are always explicit about
  what player to load a track into.
* When first loading AutoDJ, use load-and-play signal instead of
  assuming the track is loaded and ready to play when we call play().
* Make the no-deck-playing enable AutoDJ test more complete to show we
  properly transition into ADJ_IDLE mode.