Have NetworkMonitor treat as-is networks as validated.

When the captive portal app reports the network should be used
as-is (typically when there was an SSL error and the user chose
to "continue anyway via browser"), have NetworkMonitor mark the
network as validated and partial connectivity. This slightly
violates the letter of the documentation for VALIDATED, but not
the spirit.

Subsequent attempts to revalidate will also result in a result
of VALIDATED, until the network is disconnected or the captive
portal opens. Nothing triggers a new validation though, so this
will only happen if an app calls reportNetworkConnectivity.
The network is still usable in the mean time.

The validation result is sent immediately as the user taps the
"continue anyway" button. NetworkMonitor then proceeds to perform
private DNS validation, and when the full sequence of validation
is finished it will send another validation result, very likely
with the same values.

This shows a "connected" notification as the user taps on "continue
anyway via browser", which may be slightly unexpected but is
correct, so it's probably fine.

If Private DNS is in strict mode but can't be accessed after the
user decides to use as is, a notification is shown to warn the
user that the Private DNS server can't be accessed. If private
DNS settings are changed, they are reevaluated as normal.

Bug: 168431719
Test: new unit test included
      also manually tested
Change-Id: I2e6cf5f840891453cd5587a42d907c90f15f20e7