LockScreen: Fix crash when unlocking twice

This will keep track if the BroadCastReceiver is registered and
will unregister it only if so.

Change-Id: I040d710cf0927394f36b2e3da251ac5a988ce1d6
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>

Conflicts:
	policy/src/com/android/internal/policy/impl/keyguard/KeyguardSelectorView.java