Manage NAT64 prefix discovery lifecycle in the framework.

Currently NAT64 prefix discovery, which runs in netd, is started
by netd itself when a network is programmed with all-IPv6 DNS
servers. Unfortunately this is not correct because in many cases
we program DNS servers before the network is connected and it's
actually possible to send packets to them.

In general netd does not have enough visibility into network
lifecycle management to decide when to start and stop prefix
discovery. So move it into the framework with the rest of the
464xlat control plane.

Bug: 65674744
Test: atest FrameworksNetTests
Change-Id: I8fa051a9c216d9c05082bf7d0bbb0cbd56000162