Fix 5466793: Fix memory leak in system process when face lock is enabled.

Previously, the code was not unregistering the callback when we unlocked
the device, which kept a reference to LockPatternKeyguardView indirectly
by a reference to mFaceLockCallback.

It now correcly removes the callback when we unlock the device.

Change-Id: Ie592d007a1dfc2416b9e8956aba2c34e3d0120ee