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/res/layout-sw720dp/launcher.xml b/res/layout-sw720dp/launcher.xml
index 802922e..319a493 100644
--- a/res/layout-sw720dp/launcher.xml
+++ b/res/layout-sw720dp/launcher.xml
@@ -23,7 +23,7 @@
     android:layout_height="match_parent"
     android:fitsSystemWindows="true">
 
-    <com.android.launcher3.DragLayer
+    <com.android.launcher3.dragndrop.DragLayer
         android:id="@+id/drag_layer"
         android:layout_width="match_parent"
         android:layout_height="match_parent">
@@ -76,7 +76,7 @@
             android:layout_width="match_parent"
             android:layout_height="match_parent"
             android:visibility="invisible" />
-    </com.android.launcher3.DragLayer>
+    </com.android.launcher3.dragndrop.DragLayer>
 
     <ViewStub
         android:id="@+id/launcher_overlay_stub"