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