apparmor: add support for absolute root view based labels

With apparmor policy virtualization based on policy namespace View's
we don't generally want/need absolute root based views, however there
are cases like debugging and some secid based conversions where
using a root based view is important.

Signed-off-by: John Johansen <john.johansen@canonical.com>
Acked-by: Seth Arnold <seth.arnold@canonical.com>