Introduce ChromeOS-style keyboard navigation.

- Turn the DirectoryFragment (DF) and the RootsFragment (RF) into
  top-level views, and allow switching between them via the tab key.

- Disallow arrow-key navigation from switching the user between the
  DF and RF.

- When nothing is explicitly focused, make navigation keys focus the
  DF.  This makes it so that if a user opens DocumentsUI and just starts
  pressing arrow keys, they'll navigate in the directory listing.

- When restoring focus on the DF and RF, remember the last thing that
  was focused, and restore focus on that thing.

BUG=25195767
BUG=25121367

Change-Id: I00e20cbdbe9edfe269fb356440a93ef5d67c5298