Make Task have type ActivityRecord children (55/n)

ActivityRecord now extends AppWindowToken and changing Task to have
type ActivityRecord children will make the Task level merging easier.

Bug: 80414790
Test: Existing tests pass
Change-Id: I331ffe054d942cc5dd58bff369fa890b130a45f4
diff --git a/services/core/java/com/android/server/wm/AppWindowToken.java b/services/core/java/com/android/server/wm/AppWindowToken.java
index a261341..80a295d 100644
--- a/services/core/java/com/android/server/wm/AppWindowToken.java
+++ b/services/core/java/com/android/server/wm/AppWindowToken.java
@@ -1420,7 +1420,7 @@
         mReparenting = true;
 
         getParent().removeChild(this);
-        task.addChild(this, position);
+        task.addChild((ActivityRecord) this, position);
 
         mReparenting = false;