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/DragSource.java

@@ -19,11 +19,12 @@ package com.android.launcher3;
 import android.view.View;
 
 import com.android.launcher3.DropTarget.DragObject;
+import com.android.launcher3.logging.UserEventDispatcher.LaunchSourceProvider;
 
 /**
  * Interface defining an object that can originate a drag.
  */
-public interface DragSource {
+public interface DragSource extends LaunchSourceProvider {
 
     /**
      * @return whether items dragged from this source supports