[connman] Refactored counters

1. Replaced stats.c with jolla-stats.c which isn't doing anything we
   don't need. The new implementation is much smaller and simpler. It
   also eliminates the global hashtable which served the purpose of
   associating service with stats - now it's done using a pointer.
   Roaming and home statistics are separated from each other.

2. Fixed the problem with service being notified about the counter
   changes for a wrong interface. The cellular service may be associated
   with different interfaces over time. In practice it's ether rmnet0 or
   rmnet1, depending on MMS context activity and whether or not one of
   those interfaces was up when connman started (when it crashes, ofono
   interfaces stay up).
Merge pull request #18 from monich/jolla-stats

[connman] Refactored counters