Improve heart rate reading to prevent crashes

[maximDevTweet]

In HRM subview body and heart rate sensors are used. The exercise object itself provides in stats (exercise.stats.heartRate.current) the current heart rate, so this data can be directly read from the exercise without explicitly starting of heart rate sensor.
In my more complex apps I had app crashes on Versa 2, because I started the sensors and the exercises. On simulator it is displayed that the heart rate sensors are running anyway, when an exercise is started.
A fix could be to pass the exercise to heart rate subview in order to read the needed data there.