Revisit kernel setenforce

Kernel userspace helpers may be spawned running in the kernel
SELinux domain. Those userspace helpers shouldn't be able to turn
SELinux off.

This change revisits the discussion in
https://android-review.googlesource.com/#/c/71184/

At the time, we were debating whether or not to have an allow rule,
or a dontaudit rule. Both have the same effect, as at the time we
switch to enforcing mode, the kernel is in permissive and the operation
will be allowed.

Change-Id: If335a5cf619125806c700780fcf91f8602083824