KeygaurdSelectorView: Unregister reciever in onPause

Also check if we already have a reciever instance before
creating a new instance

Patch Set 2: Refactor field reciever to mUnlockReciever
           : Add null check to Context#unregisterReciever
Patch Set 3: Add null check to Vibrator#vibrate(int);
Change-Id: Ie9b03b20da29d05a64293ef6b7931393c34796a2