Support learning the NAT64 prefix from two different sources.

The NAT64 prefix from the RA always takes precedence over the
NAT64 prefix from DNS discovery, because it is detected faster,
and detecting it does not require sending any packets.

Bug: 150648313
Test: new unit test
Merged-In: Ic7452431d2d9aea1ae59b67a9d8383c6cc5b3902
Change-Id: Ic7452431d2d9aea1ae59b67a9d8383c6cc5b3902
4 files changed