[pkg-apparmor] Bug#870697: Please provide systemd service file which doesn't require the SysV init script

[intrigeri]

intrigeri:
> My initial draft seems to be working fine at first glance:

Updated my already outdated upstream MR (merge conflicts: my MR
prompted other upstream folks to take a look at the affected file… and
themselves submit a bunch of other MRs to remove obsolete bits :)
+ pinged those I'd like a review from.

Now, if these reviews take too long I'll do whatever it takes to get
this into Buster.