apparmor: remove sid from profiles

The sid is not going to be a direct property of a profile anymore, instead
it will be directly related to the label, and the profile will pickup
a label back reference.

For null-profiles replace the use of sid with a per namespace unique
id.

Signed-off-by: John Johansen <john.johansen@canonical.com>
Acked-by: Kees Cook <kees@ubuntu.com>