Add Support for App Widgets as Overlays.

This changelist adds support for loading App Widgets
from a ComponentName. This functionality is wrapped in
an OverlayProvider for integration with
DreamOverlayService.

Bug: 201675932
Test: atest AppWidgetOverlayProviderTest
Change-Id: Iddc6297ee0a9eb6c945a70368ba1f1b7ef0b1a0b