Handle provider info updates

The CommunalWidgetHost now produces a flow of AppWidgetProviderInfos
that contains the widgets bound to the host. The host also keeps a
listener for each widget and updates this flow when a provider info has
changed.

Test: atest CommunalWidgetHostTest
Test: atest CommunalAppWidgetHostTest
Test: atest CommunalWidgetRepositoryImplTest
Test: atest CommunalAppWidgetHostStartableTest
Bug: 330945453
Flag: ACONFIG com.android.systemui.communal_hub TEAMFOOD
Change-Id: I5a6477195fc351b623fac6194b3448d90f348315