Ignore empty tasks when unfreezing recents list after timeout

- The top task can sometimes be a root that doesn't have any
  children (ie. split root), which can lead to an empty
  task being in the recents list

Bug: 275425871
Test: atest SwitchBetweenSplitPairs
Change-Id: Ia0bd7a08870e8cbd4849f79506fd8db9a1d18d65

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

@@ -338,7 +338,8 @@ class RecentTasks {
         synchronized (mService.mGlobalLock) {
             final Task focusedStack = mService.getTopDisplayFocusedRootTask();
             final Task topTask = focusedStack != null ? focusedStack.getTopMostTask() : null;
-            resetFreezeTaskListReordering(topTask);
+            final Task reorderToEndTask = topTask != null && topTask.hasChild() ? topTask : null;
+            resetFreezeTaskListReordering(reorderToEndTask);
         }
     }
 

services/tests/wmtests/src/com/android/server/wm/RecentTasksTest.java

@@ -924,6 +924,11 @@ public class RecentTasksTest extends WindowTestsBase {
 
     @Test
     public void testFreezeTaskListOrder_timeout() {
+        for (Task t : mTasks) {
+            // Make all the tasks non-empty
+            new ActivityBuilder(mAtm).setTask(t).build();
+        }
+
         // Add some tasks
         mRecentTasks.add(mTasks.get(0));
         mRecentTasks.add(mTasks.get(1));