apparmor: isolate policy backwards compatibility to its own file

The details of mapping old policy into newer policy formats clutters
up the unpack code and makes it possible to accidentally use old
mappings in code, so isolate the mapping code into its own file.

This will become more important when the dfa remapping code lands,
as it will greatly expand the compat code base.

Signed-off-by: John Johansen <john.johansen@canonical.com>