Do not detect portals when DNS returns private IPs

When access points return private IPs (as defined in the NetworkMonitor
constant) in response to DNS probes, do not consider the access point as
behind a portal, but instead indicate that it has no connectivity.

This solves issues with some access points that return private IP
responses to DNS queries when they do not have internet access.

This feature is turned off by default while investigating its impact.
OEMs can force-enable it through a resource overlay:
config_force_dns_probe_private_ip_not_portal. Metrics to evaluate the
feature will be added in a later change.

Bug: 136734947
Test: atest NetworkStackTests
Change-Id: I51975e18f424e3b7265011000f073777f376e597