Fix bug with drag visualization and UserFolders.

When dragging an app shortcut, it was possible that we'd show a red rectangle
around a cell occupied by a UserFolder. This shouldn't be possible -- as soon
as that cell becomes the target drop cell, the folder should start handling
the drag and drop events.

Change-Id: I1b7a8b1aa9aeb7e2f1bd51ce8d947c06455e988f

src/com/android/launcher2/UserFolder.java

@@ -89,4 +89,10 @@ public class UserFolder extends Folder implements DropTarget {
         super.onOpen();
         requestFocus();
     }
+
+    @Override
+    public DropTarget getDropTargetDelegate(DragSource source, int x, int y, int xOffset, int yOffset,
+            DragView dragView, Object dragInfo) {
+        return null;
+    }
 }