Fix issue with both shell and launcher updating drag surface

- The shell drop target should only handle successful drops, otherwise
  either the system will create a return animation, or if the drag
  starter requests DRAG_FLAG_REQUEST_SURFACE_FOR_RETURN_ANIMATION,
  then the starter will own the surface when it is not handled

Bug: 269814838
Test: Drag an app from the taskbar to an area that does not trigger a
      split and ensure it animates back to the taskbar without
      flickering
Change-Id: I86aa03d3bef41d92cc7868c57103b03212967aed