[multi-part] Enable bidirectional sorting in DocumentsUI

* Add a header to list mode
* Add a model for sort state
* Wire them together

Bug: 22823056
Change-Id: I6acd184b69e1564fccc92538d2b47792c6c9f45a
diff --git a/res/values/dimens.xml b/res/values/dimens.xml
index 7cda341..ec03ba1 100644
--- a/res/values/dimens.xml
+++ b/res/values/dimens.xml
@@ -48,4 +48,6 @@
 
     <dimen name="autoscroll_edge_height">32dp</dimen>
 
+    <dimen name="doc_header_sort_icon_size">16dp</dimen>
+    <dimen name="doc_header_height">60dp</dimen>
 </resources>