Keyguard: Fix threading issues in KeyguardUpdateMonitor

Fixes a bug where KeyguardViewMediator called into KeyguardUpdateMonitor
on the wrong thread. Also adds assertions to make sure we only touch
mCallbacks on the right thread.

Change-Id: I9524b6c182cb70134fbc9a28df8148043fec8c1e
Fixes: 78128789
Test: Device boots successfully.