Transitions: Gone->Dreaming

Explicitly support gone to dreaming. At the moment, the lockscreen
briefly shows when the power button is pushed. This will move lockscreen
elements out of the way.

Bug: 260637747
Test: atest GoneToDreamingTransitionViewModelTest
NotificationPanelViewControllerTest KeyguardScenariosTest

Change-Id: I0e3adb6799a8144eaf810b9ee20e54b29d7d505b