Show the UMO in the hub mode UI

When media is playing or recommendations are present, the UMO will be
shown on top of the grid. It's not integrated into the layout grid
yet as that will be refactored soon.

Bug: 308638964
Bug: 304584416
Flag: ACONFIG com.android.systemui.communal_hub DEVELOPMENT
Test: atest CommunalMediaRepositoryImplTest CommunalInteractorTest MediaHierarchyManagerTest
Change-Id: Id50e313d64f313ea5ecbfb0bedb39fef9a50b449