Track NAT64 in the framework and start clatd iff NAT64 detected

Currently, both netd and clatd do NAT64 prefix detection, and we
start clatd on all IPv6-only networks regardless of whether netd
has detected a NAT64 prefix or not.

Instead, track the NAT64 prefix in the framework, and only start
clatd if the prefix is detected. This will allow us to remove
DNS64 detection in clatd, and pass the NAT64 prefix and the IPv6
address to clatd on the command line instead of clatd finding it
itself. That way, netd and the framework will always know how
464xlat is configured, and we'll be able to use that information
in netd.

Test: builds, boots
Test: atest FrameworksNetTests
Change-Id: Ida32d5760c5aecf7aeebef08fdb596291b2ce14a