Fix UMO missing on lock screen when starting from hub

If the glanceable hub claims that the UMO should show on the hub, this
always overrides the UMO on the lock screen. After UMO transitions to
and from the hub were implemented, the hub's claim to the UMO changed
to be purely based on transitions instead of scene state.

However, this meant that if you started on the hub and made it to the
lock screen through any method other than a direct keyguard transition,
the UMO would be missing from the lock screen. For example, if you
unlock on the hub then press power button twice to return to the lock
screen, or if you press power on the hub, wait 5 seconds, then press
power again to show the lock screen, the UMO will be missing.

This change adds a failsafe to ensure the hub only claims the UMO if
the hub is visible at all.

Bug: 343270202
Fix: 343270202
Test: atest CommunalTransitionViewModelTest
Flag: com.android.systemui.communal_hub
Change-Id: I52fb213d9cb9188272702f1c2e6765ba065e4105