4/n Add fallback to default task display area

... for the cases when a particular task display area target is not
specified, and/or the logic applies only to the default/fallback
task area on the display.

Bug: 152116619
Test: WM CTS and unit tests
Change-Id: I209629baada05b0615ea2874f39a2b30a95b7565
diff --git a/services/tests/wmtests/src/com/android/server/wm/TaskStackTests.java b/services/tests/wmtests/src/com/android/server/wm/TaskStackTests.java
index 6387a3b..d48e827 100644
--- a/services/tests/wmtests/src/com/android/server/wm/TaskStackTests.java
+++ b/services/tests/wmtests/src/com/android/server/wm/TaskStackTests.java
@@ -171,7 +171,7 @@
 
         // Reparent
         clearInvocations(task1); // reset the number of onDisplayChanged for task.
-        stack1.reparent(dc, true /* onTop */);
+        stack1.reparent(dc.getDefaultTaskDisplayArea(), true /* onTop */);
         assertEquals(dc, stack1.getDisplayContent());
         final int stack1PositionInParent = stack1.getParent().mChildren.indexOf(stack1);
         final int stack2PositionInParent = stack1.getParent().mChildren.indexOf(stack2);