Disallow drag to split depending on drag flag

- Propagate the drag flags used when starting the drag to the
  privileged drop target window so it can verify that
  DRAG_FLAG_START_INTENT_SENDER_ON_UNHANDLED_DRAG is specified
  before handling the drag to invoke split from a drag with a
  launch intent.

Fixes: 334934117
Test: atest DragDropControllerTests
Test: atest DragAndDropPolicyTest
Change-Id: I21f7ddf342f9db44676431accdacc2d5b6b871cd