Add aethercast support in AppArmor file

[Alberto Mardegan]

Canonical modified the AppArmor policy in order to make authercast work:

https://launchpad.net/~ci-train-ppa-service/+archive/ubuntu/stable-phone-overlay/+files/isc-dhcp_4.3.1-5ubuntu2_4.3.1-5ubuntu2.4~overlay1.diff.gz
(only the first chunk of the patch is relevant to this issue)

This was added to the package we were using in the phone overlay PPA: https://launchpad.net/~ci-train-ppa-service/+archive/ubuntu/stable-phone-overlay/+packages?batch=75&memo=75&start=75

Now we need the same in Ubports, but it would be convenient if we added this in Ubuntu, to reduce the delta.