[flexiglass] Switch scenes when keyguard enabledness changes

1 - When the keyguard becomes disabled, switches to the Gone scene (if not
  in lockdown mode)
2 - When the keyguard becomes enabled again, switches to the Lockscreen
  scene (if it was previously hidden when disabled)
3 - If the auth method becomes insecure, forgets the bit set in (1)

Fix: 348644111
Test: added unit tests
Flag: com.android.systemui.scene_container
Change-Id: If34fce812477654feac1066c839262455c38c61e