Report probe URL as redirect on 200 portals

On portals that return HTTP 200 with a login page instead of a redirect,
report the probe URL instead of null as the login URL.

NetworkMonitor already falls back to the probe URL to open the captive
portal app when the redirect URL is empty. This only affects the result
reported to ConnectivityService in handleNetworkTested, which is only
used to transmit to the NetworkAgent in onValidationStatusChanged.

This means that starting from S, NetworkAgents will also see a redirect
URL on 200 portals, instead of a failed validation (null URL) as before.

The change does not affect R and earlier as behavior of
ConnectivityService and NetworkAgents is OEM-specific and may lead to
unexpected results.

Bug: 172048052
Test: atest NetworkStackTests
Change-Id: I4c68bfc9ae8aa38d73bc62443ac923f07ee1ed46