If a VpnService only configures IPv4 addresses, routes and DNS servers, block IPv6 by default, and vice versa. Also add an API to unblock a family without needing to add an address, route or DNS server. New API with stub implementation to be filled out later. Bug: 15972465 Change-Id: I70d4d5c30ee71802610f6e16f100db6cbccef42c