Merge changes from topics "201676043", "201676597", "204605351"

* changes:
  Allow App Widget Overlays by Product Configuration.
  Add Support for App Widgets as Overlays.
  Centralized Dream Overlay State.
  SystemUI IDreamOverlay Implementation.
  Add Stub implementation for DreamOverlayService.
  Consolidate dream overlay binding logic.
  Pass Window to DreamOverlayService when attached.
  Correct launch task behind when dream is present.