[multi-part] Enable bidirectional sorting

* Add sort widget for grid mode
* Remove sort menu items
* Add a divider between header and contents
* Fix an auto-scrolling bug in drag and drop exposed by adding a header
* Add unit test for sort dimension

Change-Id: Id4f1195778d288ec0b0a95f7a0d16efb5604ba40
diff --git a/res/values/dimens.xml b/res/values/dimens.xml
index ec03ba1..cc8c244 100644
--- a/res/values/dimens.xml
+++ b/res/values/dimens.xml
@@ -50,4 +50,7 @@
 
     <dimen name="doc_header_sort_icon_size">16dp</dimen>
     <dimen name="doc_header_height">60dp</dimen>
+
+    <dimen name="dropdown_sort_widget_margin">20dp</dimen>
+    <dimen name="dropdown_sort_widget_icon_size">30dp</dimen>
 </resources>