Dispose WMS.DragInputEventReceiver on the correct thread

WindowManagerService.DragInputEventReceiver should be disposed
on the same handler where input is being handled. This is required
to avoid races between nativeFinishInputEvent and native dispose.

Bug: 26927018
Change-Id: Ib476dc0e8a8825e1df81f1557d48b619b7f9dd8c