Moving drag-drop related code into a separate package.

This is per an earlier CR comment "we should probably move all this code to its own package (launcher3.dragndrop) in a separate cl".

I'm not moving DragSource because it's referred from gsa code.

Bug: 22609426
Change-Id: Ia7204dab99c0c395c66b77143a2d60411153f5f3
diff --git a/src/com/android/launcher3/DropTarget.java b/src/com/android/launcher3/DropTarget.java
index 3748464..592cd32 100644
--- a/src/com/android/launcher3/DropTarget.java
+++ b/src/com/android/launcher3/DropTarget.java
@@ -16,6 +16,8 @@
 
 package com.android.launcher3;
 
+import com.android.launcher3.dragndrop.DragView;
+
 import android.graphics.PointF;
 import android.graphics.Rect;