Make PopupWindows into root namespaces

PopupWindows weren't marked as root namespaces. This was
causing problems for focus-related searches since they often
used isRootNamespace to detect when hitting the root of a
tree.

Bug: 38026649
Test: Ran Docs and verified that default cluster is detected
      properly in popup windows.

Change-Id: If8e1db2bb83626d04770a18c0e5ccb88b3793de4

core/java/android/widget/PopupWindow.java

@@ -1354,6 +1354,7 @@ public class PopupWindow {
         }
 
         mDecorView = createDecorView(mBackgroundView);
+        mDecorView.setIsRootNamespace(true);
 
         // The background owner should be elevated so that it casts a shadow.
         mBackgroundView.setElevation(mElevation);