SystemMonitor: implement FreeRtosMonitor only if trace facility is set

Split SystemMonitor into h and cpp file and move the logging code of the
`Process` function into the cpp file.

Depending of the `configUSE_TRACE_FACILITY` define from
`src/FreeRTOSConfig.h` create either a "FreeRtosMonitor" or a
"DummyMonitor".

Make the `Process()` function non-const, as the FreeRtosMonitor changes
the member variable `lastTick`.

In `SystemTask.h` we then only need to use `SystemMonitor`, without
knowledge of the `configUSE_TRACE_FACILITY` define.

src/systemtask/SystemTask.h

@@ -148,11 +148,7 @@ namespace Pinetime {
       bool stepCounterMustBeReset = false;
       static constexpr TickType_t batteryMeasurementPeriod = pdMS_TO_TICKS(10 * 60 * 1000);
 
-#if configUSE_TRACE_FACILITY == 1
-      SystemMonitor<FreeRtosMonitor> monitor;
-#else
-      SystemMonitor<DummyMonitor> monitor;
-#endif
+      SystemMonitor monitor;
     };
   }
 }