unsubscribe android back handler when toplevel is disposed (#13831)

#Conflicts:
#	src/Android/Avalonia.Android/Platform/AndroidSystemNavigationManager.cs

src/Android/Avalonia.Android/Platform/AndroidSystemNavigationManager.cs

@@ -4,9 +4,11 @@ using Avalonia.Platform;
 
 namespace Avalonia.Android.Platform
 {
-    internal class AndroidSystemNavigationManagerImpl : ISystemNavigationManagerImpl
+    internal class AndroidSystemNavigationManagerImpl : ISystemNavigationManagerImpl, IDisposable
     {
-        public event EventHandler<RoutedEventArgs> BackRequested;
+        private readonly IActivityNavigationService? _navigationService;
+
+        public event EventHandler<RoutedEventArgs>? BackRequested;
 
         public AndroidSystemNavigationManagerImpl(IActivityNavigationService? navigationService)
         {
@@ -14,6 +16,7 @@ namespace Avalonia.Android.Platform
             {
                 navigationService.BackRequested += OnBackRequested;
             }
+            _navigationService = navigationService;
         }
 
         private void OnBackRequested(object sender, AndroidBackRequestedEventArgs e)
@@ -24,5 +27,13 @@ namespace Avalonia.Android.Platform
 
             e.Handled = routedEventArgs.Handled;
         }
+
+        public void Dispose()
+        {
+            if (_navigationService != null)
+            {
+                _navigationService.BackRequested -= OnBackRequested;
+            }
+        }
     }
 }

src/Android/Avalonia.Android/Platform/SkiaPlatform/TopLevelImpl.cs

@@ -45,7 +45,7 @@ namespace Avalonia.Android.Platform.SkiaPlatform
         private readonly AndroidInputMethod<ViewImpl> _textInputMethod;
         private readonly INativeControlHostImpl _nativeControlHost;
         private readonly IStorageProvider _storageProvider;
-        private readonly ISystemNavigationManagerImpl _systemNavigationManager;
+        private readonly AndroidSystemNavigationManagerImpl _systemNavigationManager;
         private readonly AndroidInsetsManager _insetsManager;
         private readonly ClipboardImpl _clipboard;
         private ViewImpl _view;
@@ -155,6 +155,7 @@ namespace Avalonia.Android.Platform.SkiaPlatform
         
         public virtual void Dispose()
         {
+            _systemNavigationManager.Dispose();
             _view.Dispose();
             _view = null;
         }