Fix UMO missing on glanceable hub

Similar to ag/27798458, adds a safe-guard to ensure we show the umo on
the hub if we are idle on the hub and not transitioning.

Fixes: 339093481
Test: atest CommunalTransitionViewModelTest
Test: atest MediaHierarchyManagerTest
Flag: com.android.systemui.communal_hub
Change-Id: I38aac5758029d4bc1e75ebfd1031fd01394a9b93