Retry captiveportal check even if only one addr

We're getting some false positive results on this check and
while it was coded to try 3 times given sufficient independent addrs
the default url resolves to a single address so we'd just try once.

Rework to try again even with fewer urls to try to reduce the false
positives.

Also adds a random query param to fool proxies into not caching.

bug:9972012
Change-Id: Ib719f40ec612065ca6bcd919549fc1164506d35a