Remove DragDropController#prepareDrag()

The prepareDrag() call is used by apps to ask the system server to
create a surface for drag shadow. Now the app creates a surface by
itself, thus we can reduce the method.

Bug: 70818582
Test: com.android.server.wm.DragDropControllerTests,
      android.server.wm.CrossAppDragAndDropTests,
      manually check the drag and drop behavior on test app.
Change-Id: Iae4acbff4f9ad68faa6324beb32bf9a28dcd67be