Let prepareDrag return error when 0 size drag shadow is specified.

Since ag/2837472, creating 0 size surface resulted in raising an
exception. To avoid it, the CL lets prepareDrag return error.

Bug: 70393408
Test: com.android.server.wm.DragDropControllerTests,
      android.server.wm.CrossAppDragAndDropTests,
      manually check the drag and drop behavior on test app.

Change-Id: Ie0c477dd9e76dd5ec8b7aed6a472e24032777cc8