[connman] Fix ofono-plugin with ipv4/ipv6-connectivity

-Reset ipv4/6_method to CONNMAN_IPCONFIG_METHOD_UNKNOWN when
protocol is set ip/ipv6/dual on ofono.

-IPv6 configuration method is either CONNMAN_IPCONFIG_METHOD_FIXED
or CONNMAN_IPCONFIG_METHOD_AUTO (SLAAC).

-Revert 8d02aa4a6f3b1148ce964c88fa489d98dc7e41bd and set index
always if method is set.