Support user event logging for drag and drop

b/30039490

Supported in this CL:
- DnD: drag from container [WORKSPACE|HOTSEAT|FOLDER|ALLAPPS|WIDGETS|DEEPSHORTCUTS]
       drag to container [HOTSEAT,WORKSPACE,FOLDER,DROPTARGETS]
- Source and target can be [FOLDER_ICON, ICON, DEEPSHORTCUT, WIDGET]
- $ adb shell setprop log.tag.UserEvent DEBUG will turn on debugging

Change-Id: I0b8b879b80e6dce85bbde6e7794f9e0677832603

src/com/android/launcher3/DropTarget.java

@@ -48,9 +48,12 @@ public interface DropTarget {
         /** The view that moves around while you drag.  */
         public DragView dragView = null;
 
-        /** The data associated with the object being dragged */
+        /** The data associated with the object, after item is dropped. */
         public ItemInfo dragInfo = null;
 
+        /** The data associated with the object  being dragged */
+        public ItemInfo originalDragInfo = null;
+
         /** Where the drag originated */
         public DragSource dragSource = null;