Use DI to implement battery detection, add BatteryCutoff property

- Removed the Xamarin.Essentials package from osu.Game and added it to osu.iOS and osu.Android only.
- iOS and Android implementations use Xamarin.Essentials.Battery, while the Desktop implementation
only returns 100% battery for now.
- Added a BatteryCutoff property to PowerStatus so it can be different for each platform (default 20%, 25% on iOS)

osu.iOS/OsuGameIOS.cs

@@ -5,13 +5,30 @@ using System;
 using Foundation;
 using osu.Game;
 using osu.Game.Updater;
+using osu.Game.Utils;
+using Xamarin.Essentials;
 
 namespace osu.iOS
 {
     public class OsuGameIOS : OsuGame
     {
+        public OsuGameIOS()
+            : base(null)
+        {
+            powerStatus = new IOSPowerStatus();
+        }
         public override Version AssemblyVersion => new Version(NSBundle.MainBundle.InfoDictionary["CFBundleVersion"].ToString());
 
         protected override UpdateManager CreateUpdateManager() => new SimpleUpdateManager();
+
+        public class IOSPowerStatus : PowerStatus
+        {
+            // The low battery alert appears at 20% on iOS
+            // https://github.com/ppy/osu/issues/12239
+            public override double BatteryCutoff => 0.25;
+            public override double ChargeLevel => Battery.ChargeLevel;
+
+            public override bool IsCharging => Battery.PowerSource != BatteryPowerSource.Battery;
+        }
     }
 }

osu.iOS/osu.iOS.csproj

@@ -116,5 +116,9 @@
       <Visible>false</Visible>
     </ImageAsset>
   </ItemGroup>
+  <ItemGroup>
+  <!-- Used for obtaining battery info. -->
+    <PackageReference Include="Xamarin.Essentials" Version="1.6.1" />
+  </ItemGroup>
   <Import Project="$(MSBuildExtensionsPath)\Xamarin\iOS\Xamarin.iOS.CSharp.targets" />
 </Project>