[connman] walled garden check with http-status code 204

The current way of checking online status with http-headers is broken with number of operators removing headers with their proxies. Doing a check againts return code (204 = no content) works much more better.

This modification to work requires configuration changes: Ipv4StatusUrl to http://ipv4.jolla.com/return_204 & Ipv6StatusUrl to http://ipv6.jolla.com/return_204.