Notify of new top task changes when a top task is moved to the back

- Launcher currently relies on the current top task to cache the task
  for the next swipe gesture (skipping a synchronous binder call to
  fetch the top task, which can cause ANRs)

Bug: 232423798
Test: Open task B from task A, go back, and ensure Launcher gets
      notified that A is moved to top
Change-Id: Ia1ff90c776437c7443a472cea587b6bf78e332f0

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

@@ -451,6 +451,14 @@ final class TaskDisplayArea extends DisplayArea<WindowContainer> {
         if (!toTop) {
             if (t.mTaskId == mLastLeafTaskToFrontId) {
                 mLastLeafTaskToFrontId = INVALID_TASK_ID;
+
+                // If the previous front-most task is moved to the back, then notify of the new
+                // front-most task.
+                final ActivityRecord topMost = getTopMostActivity();
+                if (topMost != null) {
+                    mAtmService.getTaskChangeNotificationController().notifyTaskMovedToFront(
+                            topMost.getTask().getTaskInfo());
+                }
             }
             return;
         }