Step sensor may start at arbitrary value

[aaaaalbert]

A minor enhancement to consider: The step sensor

returns the number of steps taken by the user since the last reboot while activated

(see Android docs), so it may happen that a vessel's first reading of the step sensor is a large value already. That doesn't invalidate the reading, but it gives some insight into the device's usage history.

We could offset the step counter value with the (negative) value at vessel start, just as we do for the sensor event timestamps that Repy sees.