[activity_recognition_flutter 1.2.4+1] option to end activity updates

### Description
The [ActivityRecognition class](https://github.com/cph-cachet/flutter-plugins/blob/master/packages/activity_recognition_flutter/lib/activity_recognition_flutter.dart) can only start the activity updates but there does not seem to be an option to cancel the internal stream subscription. There is
[endActivityUpdates()](https://github.com/cph-cachet/flutter-plugins/blob/master/packages/activity_recognition_flutter/lib/channel/activity_channel.dart#L29) already available but it doesn't seem to be used. Could this be added or can maybe someone help me in understanding how to stop the acitivity updates?