Adding debugging and naive fix for issue 15334203

-> To be reverted, just need signed build.

Change-Id: I6a83e55067d8d2564bb3c4f0c314befbb2793f09

src/com/android/launcher3/ButtonDropTarget.java

@@ -75,6 +75,7 @@ public class ButtonDropTarget extends TextView implements DropTarget, DragContro
                 return drawables[i];
             }
         }
+        System.out.println("No drawable, returning null " + this);
         return null;
     }