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
diff --git a/services/core/java/com/android/server/wm/InputMonitor.java b/services/core/java/com/android/server/wm/InputMonitor.java
index b9c55a5..535ce9d 100644
--- a/services/core/java/com/android/server/wm/InputMonitor.java
+++ b/services/core/java/com/android/server/wm/InputMonitor.java
@@ -249,7 +249,7 @@
             if (DEBUG_DRAG) {
                 Log.d(TAG_WM, "Inserting drag window");
             }
-            final InputWindowHandle dragWindowHandle = mService.mDragState.mDragWindowHandle;
+            final InputWindowHandle dragWindowHandle = mService.mDragState.getInputWindowHandle();
             if (dragWindowHandle != null) {
                 addInputWindowHandleLw(dragWindowHandle);
             } else {