Improve battery percentage calculation and reporting

While charging, percentage should only go up, and while discharging,
percentage should only go down.

src/systemtask/Messages.h

@@ -24,7 +24,7 @@ namespace Pinetime {
         SetOffAlarm,
         StopRinging,
         MeasureBatteryTimerExpired,
-        BatteryMeasurementDone,
+        BatteryPercentageUpdated,
       };
     }
 }

src/systemtask/SystemTask.cpp

@@ -349,14 +349,10 @@ void SystemTask::Work() {
           motorController.RunForDuration(15);
           break;
         case Messages::MeasureBatteryTimerExpired:
-          sendBatteryNotification = true;
           batteryController.Update();
           break;
-        case Messages::BatteryMeasurementDone:
-          if (sendBatteryNotification) {
-            sendBatteryNotification = false;
-            nimbleController.NotifyBatteryLevel(batteryController.PercentRemaining());
-          }
+        case Messages::BatteryPercentageUpdated:
+          nimbleController.NotifyBatteryLevel(batteryController.PercentRemaining());
           break;
 
         default:

src/systemtask/SystemTask.h

@@ -133,14 +133,12 @@ namespace Pinetime {
       TimerHandle_t dimTimer;
       TimerHandle_t idleTimer;
       TimerHandle_t measureBatteryTimer;
-      bool sendBatteryNotification = false;
       bool doNotGoToSleep = false;
 
       void GoToRunning();
       void UpdateMotion();
       bool stepCounterMustBeReset = false;
       static constexpr TickType_t batteryMeasurementPeriod = pdMS_TO_TICKS(10 * 60 * 1000);
-      TickType_t lastBatteryNotificationTime = 0;
 
 #if configUSE_TRACE_FACILITY == 1
       SystemMonitor<FreeRtosMonitor> monitor;