Auto-open Captive Portal when user clicks on open network.

Rationale: many users are confused about what to do after tapping the captive
portal network in Settings, and since there is explicit user action to
connect to that network at that point, auto-opening the portal is
natural and makes it simpler.

Bug: 148538768
Test: manually with local AP with Captive Portal and:
> make RunSettingsRoboTests -j40
> atest com.android.server.ConnectivityServiceTest

Change-Id: I29573132cd3e46ff22e6f67bb3678516fabac47d