SystemUI: Visualizer tile improvements

* Set the initial visibility state, so if music is playing and the user
adds the tile, it will show up properly.

* Fix tile not hiding after pausing or closing the media player.

* Fix ConcurrentModificationException when removing tile while music is
playing

Change-Id: Ia2b246879458c05a3637266ba35d55897a76b296
Signed-off-by: Roman Birg <roman@cyngn.com>