Clear preferred top focusable root task

When the last activity in the task is finishing and there is no next
activity can be resumed. We will use pre-recorded task which is no
longer visible to resume activity. This will not only fail to resume
activity, but also miss the chance to resume home.

We should not ask an invisible task to resume activity, so this CL is to
clear mPreferredTopFocusableRootTask when we can't find a next focusable
task.

Bug: 183373468
Test: Finish SUW and use back key to finish AllSetActivity
Test: atest ActivityRecordTests TaskTest RootTaskTests
Test: atest DisplayContentTests#testNotResumeHomeRootTaskOnRemovingDisplay
Change-Id: I38b69b9dd25ac1fbf88042fb943f1200c3e33474

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

@@ -3080,6 +3080,11 @@ class Task extends WindowContainer<WindowContainer> {
             focusableTask = mRootWindowContainer.getNextFocusableRootTask(this, !allowFocusSelf);
         }
         if (focusableTask == null) {
+            final TaskDisplayArea taskDisplayArea = getDisplayArea();
+            if (taskDisplayArea != null) {
+                // Clear the recorded task since there is no next focusable task.
+                taskDisplayArea.clearPreferredTopFocusableRootTask();
+            }
             return null;
         }
 

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

@@ -143,6 +143,7 @@ final class TaskDisplayArea extends DisplayArea<WindowContainer> {
      * unable to resume target root task properly when there are other focusable always-on-top
      * root tasks.
      */
+    @VisibleForTesting
     Task mPreferredTopFocusableRootTask;
 
     /**
@@ -2128,6 +2129,10 @@ final class TaskDisplayArea extends DisplayArea<WindowContainer> {
                 && mDisplayContent.getOrientationRequestingTaskDisplayArea() == this;
     }
 
+    void clearPreferredTopFocusableRootTask() {
+        mPreferredTopFocusableRootTask = null;
+    }
+
     @Override
     protected boolean isTaskDisplayArea() {
         return true;