Enforce rounded corners for communal widgets

- Created CommunalAppWidgetHostView that subclasses AppWidgetHostView
  with support for rounded corner.
- Created CommunalAppWidgetHost that creates a CommunalAppWidgetHostView.
- Forked RoundedCornerEnforcement class from Launcher3 code.

Bug: b/318560812
Flag: ACONFIG com.android.systemui.communal_hub DEVELOPMENT
Test: manually with google tv widget
Test: atest CommunalAppWidgetHostTest
Change-Id: Ib4e2f705a070766eca605af81b38f8a88b95204c