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/ButtonDropTarget.java

@@ -203,6 +203,9 @@ public abstract class ButtonDropTarget extends TextView
                 DragLayer.ANIMATION_END_DISAPPEAR, null);
     }
 
+    @Override
+    public void prepareAccessibilityDrop() { }
+
     @Thunk abstract void completeDrop(DragObject d);
 
     @Override