Support changing the NAT64 prefix without removing it.

This cannot (currently) happen with DNS64 detection, but it can
happen with the PREF64 option.

Bug: 150648313
Test: atest ConnectivityServiceTest Nat464XlatTest --rerun-until-failure 100
Merged-In: I789fe9d46d3ac5d074ae697d23013f24a9e0246d
Change-Id: I789fe9d46d3ac5d074ae697d23013f24a9e0246d
2 files changed
tree: fe928f0461ba1b32a18b9ebd8313d7ad52368c7e
  1. core/
  2. services/
  3. tests/