frameworks/base: IPV6 tethering support

This patch adds two new API calls addUpstreamV6Interface and
removeUpstreamV6Interface to the tethering framework to pass the IPv6
upstream and downstream interfaces to netd if IPv6 is available.
Required to enable ICMPv6 forwarding.

Change-Id: I1f4f6284b2f152d8b4f0fbd02ac5a17f57c71dd7