Refactor DocumentHolder.

Primary goals of this refactor are to reduce DirectoryFragment bloat,
and to simplify the code (especially the binding code) for the different
layouts.

- Decouple DocumentHolder from DirectoryFragment.
- Move it into its own file.
- Move binding code from DirectoryFragment into DocumentHolder.
- Split DocumentHolder implementation into three separate subclasses,
  for grid items, list items, and dividers.

BUG=24326989

Change-Id: I217bf4e5b8e1b33173b8b0275591a8c5d8e9161c