Move drag related handler code to DragHandler

The CL creates new Handler in DragDropController and moved drag related
message from WindowManagerService#H and DragState#DragStateHandler.

Bug: 65564090
Test: android.server.wm.CrossAppDragAndDropTests, manually check the
      drag and drop behavior on test app.
Change-Id: Ifd0312853734468ef9ff8cf6490164a6fd8806fa
5 files changed