Reset mAssistantVisible state whenever device starts going to sleep

Fixes: 266501461
Test: atest KeyguardUpdateMonitorTest
Test: manual
  1. Enable AoD
  2. Enable face & fp
  3. Enable voice assistant
  4. Keep the device away from face
  5. Go to AoD
  6. Trigger assistant with "Ok Google"
  7. Once device wakes up and assistant is listening, say "set an alarm"
  8. Face auth will trigger, but don't unlock the device.
  9. Wait until face auth stops
  10. Press power button to go to AoD while assistant is visisble
  11. Face auth shouldn't run after the devices goes into AoD
Change-Id: Idffe08420baec1eb05b6b3f81d0381bb3cddf193