collect task moving-to-back

Otherwise it will flicker.

Bug: 281944233
Test: open chrome+gmail in split, close gmail via back.
Change-Id: Ibf3f0f98aa8dfc4476b246b6906978e4439d8ff7

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

@@ -5673,6 +5673,7 @@ class Task extends TaskFragment {
                         }
                         mTransitionController.requestStartTransition(transition, tr,
                                 null /* remoteTransition */, null /* displayChange */);
+                        mTransitionController.collect(tr);
                         moveTaskToBackInner(tr);
                     });
         } else {