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/proguard.flags b/proguard.flags
index a8e2b60..a683fe1 100644
--- a/proguard.flags
+++ b/proguard.flags
@@ -39,7 +39,7 @@
   public int getY();
 }
 
--keep class com.android.launcher3.DragLayer$LayoutParams {
+-keep class com.android.launcher3.dragndrop.DragLayer$LayoutParams {
   public void setWidth(int);
   public int getWidth();
   public void setHeight(int);