[pkg-apparmor] Bug#879585: apparmor: Pin the AppArmor feature set in Stretch to Linux 4.9's

[intrigeri]

intrigeri:
> 1. ensure the blocking kernel bug is fixed:
>    https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=883703#32

That's now done in stretch-backports: linux-latest was updated and for
example linux-image-amd64 now pulls linux-image-4.14.0-0.bpo.3-amd64
(version 4.14.13-1~bpo9+1).

So the next steps are:

2. adjust the Stretch packaging branch so it uses the same
   implementation as testing/sid (#879585)

3. update the stretch-pu request (#882697)

Cheers,
-- 
intrigeri