Pass DNS server info notifications to observers.

These are sent if the device receives IPv6 Router Advertisements
with DNS server configuration options. Currently, nothing listens
to them; in a future change we will use them as IPv6 DNS servers.

[Cherry-pick of 416740ad]

Bug: 9180552
Change-Id: I05000c0cd3867a68ab390102e8470b6912a9d3aa