[addons] cleanup the service add-on pre/post install code

@amet this is the cleanup I was referring to.

I'm not completely sure why this code was added, but this cleans it up.  I dunno what potential issues the old code was covering up.

@amet, @MartijnKaijser mind testing thoroughly?  Affects service add-on install from repo, install from zip, and upgrade.  Would be worth testing both when services are disabled or enabled (and running) prior to the action.