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

[intrigeri]

Control: severity -1 normal

1. AppArmor is not enabled by default in Stretch
2. Currently stretch-backports has Linux 4.12 that's compatible with
   the policy shipped in Stretch, without need for any pinning

⇒ lowering priority. It'll become more important once Linux 4.13+
lands in stretch-backports, but even then IMO that'll remain a normal
severity bug because a combination of 2 non-default settings is
required to trigger any problem.