Active Display: Catch possible NPE during handleDismissNotification()

* AD doesn't register if notification has been cleared on different device
  (computer) so catch the NPE that would ensue

Change-Id: Ibdb7f526d2c709ebf7a888de717247b7a6114e3e
Signed-off-by: Brandon McAnsh <brandon.mcansh@gmail.com>

Conflicts:
	packages/SystemUI/src/com/android/systemui/statusbar/policy/activedisplay/ActiveDisplayView.java