[Tizen] [multi-users] remove setuid root in xwalk-pkg-helper

xwalk-pkg-helper is always run by root user. It needs to be launched
by a user in order to retrieve User ID in application framework
and then install/remove applications in user's places.

related to XWALK-1649

Signed-off-by: Corentin Lecouvey <corentin.lecouvey@open.eurogiciel.org>