Reland "Allow UMO to show over the dream"

This reverts commit 7669ab8e.

The bug that required this CL to be reverted (b/322549132) was caused
by allowing scene changes even when the hub is not available. Relanding
this CL as-is with scene changes disallowed when the hub is not
available.

Bug: 322549132
Fixed: 322549132
Test: atest MediaHierarchyManagerTest
Test: manually verified the UMO shows up properly on the lockscreen
Flag: ACONFIG com.android.systemui.communal_hub STAGING
Change-Id: Ic38ba43f8b9d0fcfbed39ac1bf6ca5f2475d76c9