selinux: Android kernel compatibility with M userspace

NOT intended for new Android devices - this commit is unnecessary
for a target device that does not have a previous M variant.

DO NOT upstream. Android only.

Motivation:

This commit mitigates a mismatch between selinux kernel and
selinux userspace. The selinux ioctl white-listing binary policy
format that was accepted into Android M differs slightly from what
was later accepted into the upstream kernel. This leaves Android
master branch kernels incompatible with Android M releases. This
patch restores backwards compatibility. This is important because:

1. kernels may be updated on a different cycle than the rest of the
   OS e.g. security patching.
2. Android M bringup may still be ongoing for some devices. The
   same kernel should work for both M and master.

Backwards compatibility is achieved by checking for an Android M
policy characteristic during initial policy read and converting to
upstream policy format. The inverse conversion is done for policy
write as required for CTS testing.

Bug: 22846070
Change-Id: I2f1ee2eee402f37cf3c9df9f9e03c1b9ddec1929
Signed-off-by: Jeff Vander Stoep <jeffv@google.com>