Add checkmark icons for selection.

- Add a new checkmark icon.
- Modify the grid and list items to include the check icon in the
  appropriate location.
- Add code to various DocumentHolder subclasses to cross-fade between
  icons when things are selected/deselected.
- Create new dimensions quantities to represent new icon sizes.
- Clean up xml files.

BUG=24326546

Change-Id: Ib3f04c8274d35ac3b95ed13730df340e8b7c36c6
diff --git a/res/values/dimens.xml b/res/values/dimens.xml
index 060871d..cacdf4d 100644
--- a/res/values/dimens.xml
+++ b/res/values/dimens.xml
@@ -18,6 +18,10 @@
     <dimen name="icon_size">40dp</dimen>
     <dimen name="root_icon_size">24dp</dimen>
     <dimen name="root_icon_margin">0dp</dimen>
+    <dimen name="check_icon_size">30dp</dimen>
+
+    <dimen name="list_item_thumbnail_size">40dp</dimen>
+    <dimen name="grid_item_icon_size">30dp</dimen>
 
     <dimen name="grid_width">152dp</dimen>
     <dimen name="grid_height">176dp</dimen>