Inform the DNS resolver when it needs to do DNS64 synthesis.

This is only necessary when learning the NAT64 prefix from the
RA, because if the NAT64 prefix is learned from DNS, the DNS
resolver already knows the prefix and automatically enables
DNS64 synthesis.

The DNS resolver needs to be informed of the prefix any time
clat is running on a prefix learned from an RA. This is simple to
implement: just set the prefix when starting clat if prefix
discovery is not running, and clear the prefix when stopping clat
if prefix discovery was not running. This ensures that the prefix
is cleared iff it was set.

Bug: 156914456
Test: new unit test coverage
Change-Id: If8ad2d30712a6df3e207c8d3e8a129705242191e