Allow system_server to hard link its own files

Specifically, backup of wallpaper imagery needs to use hard links to
achieve "real file" access to the large imagery files without rewriting
the contents all the time just to stage for backup.  They can't be
symlinks because the underlying backup mechanisms refuse to act on
symbolic links for other security reasons.

Bug 25727875

Change-Id: Ic48fba3f94c92a4b16ced27a23646296acf8f3a5