Switch DirectoryFragment code to use model IDs.

A Model ID is a unique, stable identifier referring to a document.
The basic idea is to move away from doing everything using adapter
positions, and instead use model IDs.

This is the first in a line of CLs toward that goal.  It does the
following:

- Introduce the concept of a Model ID, which is unique for each
  document.
- Add a method to retrieve items from the Model using ID rather than
  adapter position.
- Transition code in the DirectoryFragment to talk to the Model using
  Model IDs rather than positions.
- Break the Model class out into a separate file of its own.

BUG=26024369

Change-Id: Ia5171f089d6b8a83855423ec05cf14dbfc7b6ba8