Fix face auth not running as expected when flexiglass is enabled.

When flexiglass is enabled, `keyguardRepository.isKeyguardGoingAway` is not the source of truth anymore for whether lockscreen is going away.

Changing it to rely on transition to UNDEFINED.

Fixes: 353453872
Fixes: 357668029
Test: atest DeviceEntryFaceAuthRepositoryTest
Test: manually, enable face auth, enable flexiglass, go to lockscreen, go to bouncer and swipe up, face auth should run.
Flag: com.android.systemui.scene_container
Change-Id: I5694044ac5a718fe3842e9717906fd597a803003