Unhide NETLINK_NETFILTER constant

Tethering use NETLINK_NETFILTER to update NAT conntrack entry for
tethering offload. Netlink netfilter socket is protected by SELinux
that only system_server, netd and networkStack(tethering) may use it.

Bug: 144814535
Test: build

Change-Id: I40eedc89d72cf0a1fcdf670b453ef034b2e7b13c