Fix the WriteStringToFile overload that takes mode/owner/group.

The actual bug is == instead of !=, but the real cause was me trying to be
too clever. This patch switches to much simpler code, and -- since the
intended use of this code is security anyway -- adds logging if anything
goes wrong.

Bug: 19361774
Change-Id: If2af07d31a5002f9010b838247b691f6b28bdfb1