Always start dream underneath hub

When we transition fully to the hub, we start the dream underneath the
hub to allow the user to swipe away the hub and enter the dream.

Test: atest CommunalDreamStartableTest
Flag: ACONFIG com.android.systemui.communal_hub TEAMFOOD
Bug: 325102385
Change-Id: Ic3f0a18a28858925759342230bf47f588f4f9836