Keep top fullscreen tasks on top when dismiss splits

A fullscreen task was added on top and exit split-screen mode
while an activity started, but the activity didn't be resumed
because the belonging task was no longer on top after split-screen
tasks were reparented to top.

Bug: 161170237
Test: start emergency dialer when split-screen activated

Change-Id: I44dbe0d2966dd831264ca366f5889d6429d78972

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

@@ -1324,7 +1324,13 @@ final class TaskDisplayArea extends DisplayArea<Task> {
     }
 
     void onSplitScreenModeDismissed() {
-        onSplitScreenModeDismissed(null /* toTop */);
+        // The focused task could be a non-resizeable fullscreen root task that is on top of the
+        // other split-screen tasks, therefore had to dismiss split-screen, make sure the current
+        // focused root task can still be on top after dismissal
+        final Task rootTask = getFocusedStack();
+        final Task toTop =
+                rootTask != null && !rootTask.inSplitScreenWindowingMode() ? rootTask : null;
+        onSplitScreenModeDismissed(toTop);
     }
 
     void onSplitScreenModeDismissed(Task toTop) {