Improve transition between hub and edit mode

This change improves the transition between hub and edit mode by
introducing a background in the SystemUI hub to hide the edit mode
activity launching and exiting animations, making the visible transition
much cleaner.

Test: atest CommunalViewModelTest
Test: atest CommunalEditModeViewModelTest
Test: atest FromPrimaryBouncerTransitionInteractorTest
Test: atest FromAlternateBouncerTransitionInteractorTest
Test: manual; see recordings in bug
Bug: 419361764
Flag: com.android.systemui.hub_edit_mode_transition
Change-Id: Ia366ce34d88acc74b7f063e5f8710415ba49e9cd