Fix ConnectivityServiceTest for NATT Keepalive

Fix test breakages I caused when adding cell
support for NATT keepalives.

-Make the minimum keepalive interval a constant in
 ConnectivityManager and use it in tests.
-Re-Disallow IPv6 Keepalives

Bug: 73327535
Test: 'runtest -x ConnectivityServiceTest' now passes
Change-Id: I5ec4367d250ee371014e65c897c3897a25a05e2d