[CLATJ#8] ClatCoordinator: select IPv6 address with checksum-neutral IID

Select a local 464xlat IPv6 address for clatd. The IPv6 address has
checksum-neutral IID. The native function is moved from netd to
libclat. Its test is moved from netd as well.

Bug: 212345928
Test: flash and boot
Run "atest ClatCoordinatorTest" in a followup commit.

Change-Id: I11520e5c086e475999847d829dd6341b14199b39
2 files changed