Use the carrier-given dns addrs for tethering.

This fixes a complaint from carriers (that we used 8.8.8.8), but also
fixes the case where there is only room for one live radio
connection: the secondary connection (tethering) doesn't have a
default route to prevent on-device traffic from slipping out on the
tethering connection, but tethered dns is proxied through dnsmasq, so
it is appearing as on-device traffic and is unroutable.  By switching
to the carrier-indicated dns servers we can use the host-routes
already set for those and kill two bugs with one fix.

bug:5898904
Change-Id: Ida8777687994f353b2d4f2c7db5d6ea4b6ac3882