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/tests/wmtests/src/com/android/server/wm/TaskStackContainersTests.java b/services/tests/wmtests/src/com/android/server/wm/TaskStackContainersTests.java
index 92ddb35..eef680b 100644
--- a/services/tests/wmtests/src/com/android/server/wm/TaskStackContainersTests.java
+++ b/services/tests/wmtests/src/com/android/server/wm/TaskStackContainersTests.java
@@ -54,7 +54,7 @@
         // Stack should contain visible app window to be considered visible.
         final Task pinnedTask = createTaskInStack(mPinnedStack, 0 /* userId */);
         assertFalse(mPinnedStack.isVisible());
-        final AppWindowToken pinnedApp =
+        final ActivityRecord pinnedApp =
                 WindowTestUtils.createTestAppWindowToken(mDisplayContent);
         pinnedTask.addChild(pinnedApp, 0 /* addPos */);
         assertTrue(mPinnedStack.isVisible());