HR Sensor : Add start/stop button to the HeartRate app (the HR sensors stays ON when the app is closed), display the HR value on the Clock app.

src/components/heartrate/Ppg.h

@@ -14,6 +14,7 @@ namespace Pinetime {
       float HeartRate();
 
       void SetOffset(uint16_t i);
+      void Reset();
 
     private:
       std::array<int, 200> data;