[connectionagent] try harder to find a wlan. this adds a scan timer

This helps autoconnect when coming from cell network back into wlan
range. connman's own background scan can take up to 5 minutes (or longer)

This timer does not run when defaultRoute is wifi.

This value can be changed in .config/nemomobile/connectionagent.conf

If this value is 0, no additional helper scanning is done.