ANDROID: selinux: modify RTM_GETNEIGH{TBL}

Map the permission gating RTM_GETNEIGH/RTM_GETNEIGHTBL messages to a
new permission so that it can be distinguished from the other netlink
route permissions in selinux policy. The new permission is triggered by
a flag set in system images T and up.

This change is intended to be backported to all kernels that a T system
image can run on top of.

Bug: 171572148
Test: atest NetworkInterfaceTest
Test: atest CtsSelinuxTargetSdkCurrentTestCases
Test: atest bionic-unit-tests-static
Test: On Cuttlefish, run combinations of:
    - Policy bit set or omitted (see https://r.android.com/1701847

)
    - This patch applied or omitted
    - App having nlmsg_readneigh permission or not
  Verify that only the combination of this patch + the policy bit being
  set + the app not having the nlmsg_readneigh permission prevents the
  app from sending RTM_GETNEIGH messages.

Change-Id: I4bcfce4decb34ea9388eeedfc4be67403de8a980
Signed-off-by: Bram Bonné <brambonne@google.com>
(cherry picked from commit fac07550bdac9adea0dbe3edbdbec7a9a690a178)
(cherry picked from commit 32d7afd1472c3cce509a3455b40a575540eac780)