Instead of explicitly updating the icon cache use the helper class that also
forces a loader update at the same time. This eliminates the possibility of
updating the icon cache without any gdk-pixbuf loaders.
Also check that the Sato icon theme isn't already set to avoid appending to the
file every time the postinst runs.
[YOCTO #2399]
Signed-off-by: Ross Burton <ross.burton@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
