Prevent flicker when starting a drag.

Also, animate item the view to its new position when dragging.
diff --git a/src/com/android/launcher2/DragSource.java b/src/com/android/launcher2/DragSource.java
index 7c6ca58..11cdcc9 100644
--- a/src/com/android/launcher2/DragSource.java
+++ b/src/com/android/launcher2/DragSource.java
@@ -24,5 +24,12 @@
  */
 public interface DragSource {
     void setDragController(DragController dragger);
+
+    /**
+     * Callback from the DragController when it begins drawing the drag view.
+     * This allows the DragSource to dim or hide the original view.
+     */
+    void onDragViewVisible();
+
     void onDropCompleted(View target, boolean success);
 }