Return to dream from occlusion.

This changelist tracks the dream state when the lockscreen is occluded
and then returns to that state on unocclusion.

Test: atest KeyguardTransitionScenariosTest#dreamingToOccludedToDreaming
Test: atest CommunalDreamStartableTest#CommunalDreamStartableTest#restartDreamingWhenTransitioningFromDreamingToOccludedToDreaming
Fixes: 338051457
Flag: ACONFIG com.android.systemui.restart_dream_on_unocclude disabled
Change-Id: If075685b466aa4a4ccfd77a757a7d715ccf5d3e3