SystemUI: KeyGuardMonitor: Prevent ConcurrentModificationException

* Instead of accessing the original ArrayList, which might be modified
  while processing the loop, create a copy of the list and work with that
  one
* This ensures that no exception is thrown when some other part unregisters
  its listener while the list is already being processed

BUGBASH-1289

Change-Id: Ie8404d88c7c87c4ec8fcd64fdd5218b25806f24f