Add a11y action for expand / collapse items in WidgetsFullSheet

Test: Enable talkback. When focusing on a collapsed item, it announces
      Collapsed "App Name" "n widgets/shortcuts". Action available. Use
      tap with 3 fingers to view. 3 fingers tap > Action > Expand.
      Widgets are expanded.
      When focusing on an expanded item, it announces
      Expanded "App Name" "n widgets/shortcuts". Action available. Use
      tap with 3 fingers to view. 3 fingers tap > Action > Collapse.
      Widgets are collapsed.
Bug: 183120145
Change-Id: Ibbb6025b929c141aa9e2fd2c72dce314e8be837e