Update DREAM->HUB transition to use scene as source-of-truth

Instead of manually starting the KeyguardState transition, we rely on
the scene change to trigger the transition in KeyguardState. This
ensures the scene is the one driving the transition, therefore keeping
the two in sync.

Bug: 336294198
Test: tap to exit dream with glanceable hub enabled
Test: atest KeyguardTransitionScenariosTest
Flag: ACONFIG com.android.systemui.communal_hub TEAMFOOD
Change-Id: I46d7e070b8cc662c2e4bc845d59743c7fbbb6eb3