Always expire servers when receiving a zero-lifetime RDNSS option

When receiving a zero-lifetime RDNSS option for DNS servers that
is already configured, the code first sets the lifetime of the
servers to zero, and then runs code that prunes expired servers.

Because expired servers are pruned only if "expiry < now", if the
pruning happens in the same millisecond as the update, the
servers aren't actually pruned. This can happen multiple times,
so if the code runs fast enough, the server will never expire.
Fix this by using <= instead of <.

Fix: 143806550
Test: makes the new IpClientIntegrationTest#testRaRdnss not flaky
Change-Id: Icf6893efdc028859b178eb234f6f1a42b24e2936