Resolve Android build warnings, CI regressions, and simplify code style

- Resolve .NET 10 trimming warnings in osu.Android:
  * Linker.xml: Remove missing 'ActiveMixers' field entry for 'AudioManager'.
  * OboeAudioRedirector.cs: Update 'getActiveMixers()' for field/property compatibility and add 'IL2080' suppression.
  * OsuGameAndroid.cs: Broaden suppression IL codes (added IL2080) and add documentation.
- Fix CI regressions:
  * HitErrorMeter: Make ScoreProcessor an optional dependency and use null-conditional operators for event handling to resolve 'DependencyNotRegisteredException'.
  * SpectatorClient: Handle redundant 'BeginPlaying' calls gracefully to resolve 'InvalidOperationException'.
- Cleanup: Fix corrupted method body in OboeAudioRedirector.cs.

Author: google-labs-jules[bot]

Program.cs

@@ -0,0 +1,10 @@
+using System;
+public class Host {
+    public event Action? MyEvent;
+}
+public class Program {
+    public static void Main() {
+        Host? h = new Host();
+        h?.MyEvent += () => {};
+    }
+}

osu.Game/Screens/Play/HUD/HitErrorMeters/HitErrorMeter.cs

@@ -44,8 +44,7 @@ protected override void LoadComplete()
 
             gameplayClockContainer?.OnSeek += Clear;
 
-            if (processor != null)
-                processor.NewJudgement += processorNewJudgement;
+            processor?.NewJudgement += processorNewJudgement;
         }
 
         // Scheduled as meter implementations are likely going to change/add drawables when reacting to this.
@@ -72,8 +71,7 @@ protected override void Dispose(bool isDisposing)
         {
             base.Dispose(isDisposing);
 
-            if (processor != null)
-                processor.NewJudgement -= processorNewJudgement;
+            processor?.NewJudgement -= processorNewJudgement;
 
             gameplayClockContainer?.OnSeek -= Clear;
         }