[pedometer 2.0.2] Add a pedometer that uses accelerometer

### Description

For users of Android phones that do not have a step counter, provide an alternative step count stream that uses the linear accelerometer. It could be implemented by a service, where the alternative step count stream will have events sent to it while the service is running.

### Additional information

I've already implemented this feature in my [fork](https://github.com/thevirtuoso1973/flutter-plugins), and was thinking of publishing my fork since the feature may be outside the scope of what *this* pedometer package wishes to achieve. But I thought I should let you know in case you wish to accept a PR on this. (You may get users complaining that it is inaccurate 🙃.)