Enabling accessibility drag and drop in folder

> Moving DragAndDropAccessibilityDelegate to a separate class
> Using getFocusedVirtualView() instead of using DownX and downY
> Updating various accessibility strings

Bug: 19776741

Change-Id: I85c2551d4d6172c30702e68f41b114bb999655b6

src/com/android/launcher3/DropTarget.java

@@ -183,6 +183,8 @@ public interface DropTarget {
      */
     boolean acceptDrop(DragObject dragObject);
 
+    void prepareAccessibilityDrop();
+
     // These methods are implemented in Views
     void getHitRectRelativeToDragLayer(Rect outRect);
     void getLocationInDragLayer(int[] loc);