firewall: periodically re-resolve NTP servers

Google's Public NTP servers don't have static IP addresses, so the
addresses that clients resolve for them will change over time. Make sure
our firewall rules stay in step with this, at least most of the time.

Fixes b/29131559.

Change-Id: Ia27a36471fb39e8cceb2797d216f148ae38177c0
3 files changed