Fix empty state hub showing briefly when swiping from lock screen

If only start listening once the hub scene transition is committed, the
empty state may show very briefly when swiping open quickly. By
listening to the transition state instead of the scene state, we can
react sooner and load widgets before the hub shows up.

Bug: 411467094
Fixed: 411467094
Test: atest CommunalAppWidgetHostStartableTest
Flag: com.android.systemui.restrict_communal_app_widget_host_listening
Change-Id: I635c03683877af4d37e835f4f0dc237a783ddd36