Move biometric setting observer from KeyguardUpdateMonitor to BiometricService

Fixes: 116872423

Test: with additional logging, do
      adb shell settings put secure face_unlock_keyguard_enabled 1 (or 0)
Test: content observer updates when user changes
Test: clients receive the current state upon registering

Change-Id: Id37381a8c263b29a0b91abb5241d74bb52364d63