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

src/com/android/launcher3/AnotherWindowDropTarget.java

@@ -27,7 +27,7 @@ import android.graphics.Rect;
 public class AnotherWindowDropTarget implements DropTarget {
     final Launcher mLauncher;
 
-    AnotherWindowDropTarget (Context context) { mLauncher = (Launcher) context; }
+    public AnotherWindowDropTarget (Context context) { mLauncher = (Launcher) context; }
 
     @Override
     public boolean isDropEnabled() { return true; }