Add delete key support.

- Pull the action mode up from the ActionMode listener into the
  DirectoryFragment.  There is no way to globally check on action mode,
  so fragment-level coordination on the action mode requires a
  fragment-level variable.

- Add a key handling clause for the delete key.  Unfortunately this
  can't be set up as a shortcut key, because shortcuts all have to be
  modified with the "Ctrl" key.

BUG=27409328

Change-Id: I48d1c87613ee7434728828e3b745fadb3bc915c1