Update grid items to look more like the mocks.

- Create a new layout for directory items, which have a different
  layout (no thumbnail, size or mod_date).

- Add drop shadows.

Also refactor a few things in the DocumentHolder and child classes to
make things more efficient and cleaner.

BUG=24326989,26229570

Change-Id: I05df52b071667190d4c4c671f50d25498383cdaa