Fix the problem that system apps cannot drag

If an app with a uid of Process.SYSTEM_UID calls view.startDragAndDrop, IllegalStateException will be thrown in Session.performDrag, which makes the drag-and-drop feature unavailable in this app.
And we don't need this check either, because validateAndResolveDragMimeTypeExtras and validateDragFlags are indeed called before Binder.clearCallingIdentity.

Bug: 245626682

Change-Id: I1c27287e51a956284dae0f61bfa0cac14dd28a27