Use new signal for mediaHostVisible

This change uses a different signal for isMediaHostVisible when
media_controls_in_compose is enabled. This prevents a crash when going
to glanceable hub and scene container is enabled.

It also moves the mediaHost initialization code into a startable rather
than in the constructor of a view model.

Test: atest CommunalViewModelTest
Test: atest CommunalMediaStartableTest
Fix: 441564893
Flag: com.android.systemui.media_controls_in_compose
Change-Id: I1d0d795c7d1d4dc50c063b1ae8701b592878368e