Send a warning notification if device is unplugged and low battery

- Uses Xamarin.Essentials in osu.Game.PlayerLoader to check battery level
- Encapsulated battery checking in the public BatteryManager class so battery level and plugged in status can be accessed and edited in TestPlayerLoader
- When checking battery level, catch NotImplementedException thrown by Xamarin.Essentials.Battery on non-mobile platforms
- Added visual unit tests for battery notification
  To mock battery status and level, we had to define a batteryManager object in TestPlayerLoader and add a new function ResetPlayerWithBattery()

Co-Authored-By: Marlina José <marlina@umich.edu>

osu.Game/Configuration/SessionStatics.cs

@@ -16,6 +16,7 @@ namespace osu.Game.Configuration
         {
             SetDefault(Static.LoginOverlayDisplayed, false);
             SetDefault(Static.MutedAudioNotificationShownOnce, false);
+            SetDefault(Static.BatteryLowNotificationShownOnce, false);
             SetDefault(Static.LastHoverSoundPlaybackTime, (double?)null);
             SetDefault<APISeasonalBackgrounds>(Static.SeasonalBackgrounds, null);
         }
@@ -25,6 +26,7 @@ namespace osu.Game.Configuration
     {
         LoginOverlayDisplayed,
         MutedAudioNotificationShownOnce,
+        BatteryLowNotificationShownOnce,
 
         /// <summary>
         /// Info about seasonal backgrounds available fetched from API - see <see cref="APISeasonalBackgrounds"/>.