Don't start face auth when device is going to sleep

Fixes an issue where KeyguardUpdateMonitor would be notified that
Keyguard is no longer going away before it was notified that the device
was going to sleep. This caused Keyguard to begin listening for face
authentication when the device was going to sleep.

Test: Manual

Fixes: 189475610
Change-Id: Ia73a6cb921828c43eb2b80b5c4686697b75aca5d