Zero out HRM when stopped

I'm working on graph data persistence for HRM and Battery for https://github.com/xan-m/Infini-iOS and I noticed last night that if the HRM is stopped on InfiniTime (i.e. if it was running, but you press the stop button), it retains the last heart rate value and continues to report it over the BLE HRM characteristic as the "current" heart rate on every connection/reconnection.

In my case last night, I had the watch just barely out of range of my phone with my iOS app open with the HRM stopped on the watch, so it was disconnecting and reconnecting pretty frequently. Every time the watch and phone were able to establish a connection, the watch reported the HRM value that it had been stopped at, and it was saved to the graph. I ended up with close to 30 reports of the heart rate value that I had stopped the HRM at.

I've got a kludgy workaround in place in my app to keep these values from being reported, but it potentially sacrifices the occasional real data point to reduce saturation of false data points. If InfiniTime zeroed this value out instead, it would be much easier to filter out (hopefully nobody's heart rate is 0...)

I realize this is an edge case, but I don't see much utility in retaining the last heart rate value after stopping the HRM. If it is useful to others, perhaps a "pause" and a "stop" button could be implemented so that users have the option to retain or clear the value at their discretion.

Trace the InfiniTime HRM stop handling and the BLE HRM characteristic reporting path. Reproduce stopping the HRM followed by a reconnect, then verify that the stopped state reports zero rather than the last heart rate and no longer creates repeated stale readings.