garmin_fit: add handling of global UTC offset

garmin_fit: add handling of global UTC offset

Some devices may record timestamps in system time (i.e. boot time),
rather than system time. As mentioned in Github issue #54 and in the
upstream documentation, we can recognize such a scenario when
timestamp is < 0x10000000.

Field 4 under the global ID for device settings records the difference
between system time and UTC time (this is called global UTC offset in
the upstream documentation).

Add code to store the global UTC offset in a new global variable, and
correctly handle all timestamps with the new information.