Remove more dead code from KeyguardUpdateMonitor

1. DisplayClientState is not used anywhere.
2. mUserFaceUnlockRunning is never updated because
   com.android.facelock.FACE_UNLOCK_STARTED/STOPPED
   intents are used/started anywhere. This seems to be
   code untouched for a long time.

Test: atest KeyguardUpdateMonitorTest passes
Bug: 238187419
Change-Id: Ifc692770f11d367a2dd3466f55fb61d482b22be8