[connman] Increase maximum size of stats files.

The default maximum size of the stats files was 16 kB. However, now
that each record is approximately twice as big (due to the change from
32 bit to 64 bit data types) the number of records which can be stored
has halved. To compensate increase the default maximum size to 32 kB.
[connman] Fix erroneous data counters when roaming.

The home and roaming pointers in the stats file header are not updated
when summarising the history file. This can result in the home or
roaming pointer pointing to an invalid record. Fixed by setting both
values to UINT_MAX (invalid) when the history file is updated.
Merge pull request #76 from amccarthy/master

Data counter fixes