Fix CrossAppDragAndDropTests failed

Currently, InputDispatcher could report the invalid drop target when
receving an unexpected cancel or the drag window has gone. In other
hand,`View.cancelDragAndDrop` could directly cancel the current drag
state cause `reportDropWindow` access the invalid state.

We should check the valid drag state when called `reportDropWindow`.

Test: atest DragDropControllerTests CrossAppDragAndDropTests
Bug: 234889646
Change-Id: Ibe9c94d2ff4471bfc5ff32d62e4701ef9a0d75d0

services/core/java/com/android/server/wm/DragDropController.java

@@ -342,6 +342,11 @@ class DragDropController {
     }
 
     void reportDropWindow(IBinder token, float x, float y) {
+        if (mDragState == null) {
+            Slog.w(TAG_WM, "Drag state is closed.");
+            return;
+        }
+
         synchronized (mService.mGlobalLock) {
             mDragState.reportDropWindowLock(token, x, y);
         }