Prevent repeatedly resuming/pausing activities in split

The embedding activity was repeatedly pausing/resuming
when the app is targeting SDK before Q. That's because
we were only comparing the z-order of the root task.
However, both of the two activities in split are in the
same task.

Update the topmost activity by comparing the z-order of
the activities.

Bug: 196923917
Test: start app with targetSDK prior Q in split
Change-Id: Ib6ffb12439689f38e83e320a0c9e739829e0f8fc

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

@@ -730,11 +730,14 @@ public class WindowProcessController extends ConfigurationContainer<Configuratio
             canUpdate = true;
         }
 
-        // Compare the z-order of ActivityStacks if both activities landed on same display.
-        if (display == topDisplay
-                && mPreQTopResumedActivity.getRootTask().compareTo(
-                        activity.getRootTask()) <= 0) {
-            canUpdate = true;
+        // Update the topmost activity if the activity has higher z-order than the current
+        // top-resumed activity.
+        if (!canUpdate) {
+            final ActivityRecord ar = topDisplay.getActivity(r -> r == activity,
+                    true /* traverseTopToBottom */, mPreQTopResumedActivity);
+            if (ar != null && ar != mPreQTopResumedActivity) {
+                canUpdate = true;
+            }
         }
 
         if (canUpdate) {