Add support for roaming/home network statistic

Add support for roaming/home network statistic

Currently connman only has one set of counters for
collecting statistics on online time and
the amount of transfered bytes.

For 3G connections we should destinguish between
home network and roaming. This patch introduces
two sets of counter values for home network
and roaming network.

Changing from Home to Roaming counters relies on
connman_network_set_roaming being called. Currently,
this is only done in the ofono plugin. Getting Wifi
'Hot-Spot Provider' Roaming (e.g.  T-Mobile Germany
to T-Mobile USA) is not yet working.