Notify battery level every 10 minutes when connected to a BLE host.

Refactor battery percent : only use uint8_t to store the battery % remaining.

src/systemtask/SystemTask.h

@@ -135,6 +135,8 @@ namespace Pinetime {
       void GoToRunning();
       void UpdateMotion();
       bool stepCounterMustBeReset = false;
+      static constexpr TickType_t batteryNotificationPeriod = 1000 * 60 * 10; // 1 tick ~= 1ms. 1ms * 60 * 10 = 10 minutes
+      TickType_t batteryNotificationTick = 0;
 
 #if configUSE_TRACE_FACILITY == 1
       SystemMonitor<FreeRtosMonitor> monitor;