SystemUI: more consistent visualizer logic

- only call setVisible from screen on and screen off methods
- eagerly unregister self to make sure we don't get added twice as a
  listener
- Add some debugging under the DEBUG flag

Change-Id: Iada13058f87d4c8d8c8b60f6eebf055652ff2c8d
Signed-off-by: Roman Birg <roman@cyngn.com>