Implement a custom ItemAnimator to enable focus to persist across change
animations in the DirectoryFragment's RecyclerView.
Note that this is a temporary stop-gap - a better fix would be to have
the RecyclerView restore focus to the appropriate item after a change
animation.
BUG=24802917
Change-Id: I4e4e6f758a2ea87e6235533860863c822d71a27a
e48e4ca564