base

Drag thumbnail fixes / improvements

* Properly wipe the thumbnail canvas before handing it to the app for
  its contents to be drawn

* Provide a getView() method in DragThumbnailBuilder that allows
  clients or subclasses to get at the associated view.  This is
  especially for clients that want to draw entire (sub)layouts as
  the drag thumbnail, by overriding onDrawThumbnail(Canvas c)
  like this:

  // Override specifically for drawing a whole ViewGroup into
  // the drag thumbnail canvas
  @Override
  public void onDrawThumbnail(Canvas c) {
      getView().dispatchDraw(c);
  }

Change-Id: Ib43ddd7cf1d44faf2d7f6ba79f102bc3c7f14596