Add and init media hosts.

This prevents a crash when expanding the shade in lockscreen, as it
expects to have QQS and QS media hosts. They are not attached to
anything yet, but eventually they will be used for media composables.

Also, fix an incorrect `isFullyCollapsed`. Now it matches the logic in
QSImpl.

Test: manual
Bug: 353253280
Flag: com.android.systemui.qs_ui_refactor_compose_fragment
Change-Id: I89fb43fd7b4264f82fa15eb49b2234af4af9ae25