Make DragDropController#mDragState private

Before introducing the drag and drop callback, we need a write lock to preserve
the order of drag state update.

The CL changes the visibility of mDragState to private so that we can
ensure the upcoming write lock guards all updates for mDragState.

Bug: 65564090
Test: android.server.wm.CrossAppDragAndDropTests, manually check the
      drag and drop behavior on test app.
Change-Id: I3afc06221e4a9688b417768365e87b19706f9372