Initialize SystemTask, DisplayApp and HeartRateTask as global static variable instead of variables on the heap. We don't need them on the heap as we know their size at build time, it'll reduce memory fragmentation and it'll make memory analysis easier.

src/components/heartrate/Ppg.h

@@ -8,8 +8,7 @@ namespace Pinetime {
   namespace Controllers {
     class Ppg {
     public:
-      explicit Ppg(float spl);
-
+      Ppg();
       int8_t Preprocess(float spl);
       float HeartRate();