Only listen to AppWidgetHost if communal is showing

Didn't notice any significant latency when doing this on tablet +
mobile, but wrapped in a flag anyways to test the behavior.

Bug: 406044666
Test: atest CommunalAppWidgetHostStartableTest
Flag: com.android.systemui.restrict_communal_app_widget_host_listening
Change-Id: Iaf357542d72f0c9152d4fe5c5e5673eaec9ad572