Refactoring ItemInfo

> Changing dragObject to ItemInfo
> Removing dropPos which is always null
> Removing requiresDbUpdate which is only used in CellLayout

Change-Id: I753ddaae0880c8a9bfee5a1266095ff34610284a

src/com/android/launcher3/DropTarget.java

@@ -47,7 +47,7 @@ public interface DropTarget {
         public DragView dragView = null;
 
         /** The data associated with the object being dragged */
-        public Object dragInfo = null;
+        public ItemInfo dragInfo = null;
 
         /** Where the drag originated */
         public DragSource dragSource = null;