Implement type-to-focus in the DirectoryFragment.

Add a helper to the FocusManager, to handle alphanumeric keystrokes.
- Build a search index mapping document titles to adapter positions, and
  keep it up-to-date.
- Intercept alphanumeric keystrokes and build up a search string from
  them.
- Use the search string and index to set focus in the directory listing.
- Highlight code is written, but not activated in the UI for now, to
  minimize surface area for potential breakages.  Revisit enabling
  highlights for O.

BUG=24988911

Change-Id: I148ff5583c655338d12bc06d6484a9ed7aa528ad