Purely rely on systemctl to stop and start the ofono service.
This is much cleaner as tests don't have to bother killing the process
manually any more.

The LD_PRELOAD is now handled by generating a temporary systemd service
config file containing the 'Environment' key.

Apertis: https://phabricator.apertis.org/T1296



Signed-off-by: Guillaume Desmottes <guillaume.desmottes@collabora.co.uk>
Reviewed-by: Emanuele Aina <emanuele.aina@collabora.com>
Differential Revision: https://phabricator.apertis.org/D5310