[connman] ofono: set ipconfig method to unknown

When set_connected is called, method needs to be set
CONNMAN_IPCONFIG_METHOD_UNKNOWN so that no old configuration
is used in case it was defined by any earlier active context and
it is not defined by the current active context.
[connman] ofono: Set CONNMAN_IPCONFIG_METHOD_AUTO when no address defined

IPv6 configuration method is either CONNMAN_IPCONFIG_METHOD_FIXED
(ipv6-pdp) or CONNMAN_IPCONFIG_METHOD_AUTO (SLAAC/DHCPv6,
dual/ipv4v6-pdp).