Tethering offload: add/remove IPv6 forwarding rules on ND events.

Use IpNeighborMonitor to listen for ND cache events on the
downstream interface, and push downstream IPv6 fowarding rules
to netd.

Rules are pushed when:
- IPv6 neighbours appear/disappear on the downstream interface.
- The upstream changes.

Test: new unit test
Change-Id: I7b01ba179a4d6bb248fd6c4994e48800613a4efa