misc: Use a few static helpers for Avalonia objects

src/Ryujinx/UI/ViewModels/DownloadableContentManagerViewModel.cs

@@ -93,10 +93,7 @@ namespace Ryujinx.Ava.UI.ViewModels
 
             _applicationData = applicationData;
 
-            if (Application.Current.ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
-            {
-                _storageProvider = desktop.MainWindow.StorageProvider;
-            }
+            _storageProvider = RyujinxApp.MainWindow.StorageProvider;
 
             LoadDownloadableContents();
         }

src/Ryujinx/UI/ViewModels/Input/InputViewModel.cs

@@ -245,9 +245,7 @@ namespace Ryujinx.Ava.UI.ViewModels.Input
         {
             if (Program.PreviewerDetached)
             {
-                _mainWindow =
-                    (MainWindow)((IClassicDesktopStyleApplicationLifetime)Application.Current
-                        .ApplicationLifetime).MainWindow;
+                _mainWindow = RyujinxApp.MainWindow;
 
                 AvaloniaKeyboardDriver = new AvaloniaKeyboardDriver(owner);
 

src/Ryujinx/UI/ViewModels/ModManagerViewModel.cs

@@ -86,10 +86,7 @@ namespace Ryujinx.Ava.UI.ViewModels
 
             _modJsonPath = Path.Combine(AppDataManager.GamesDirPath, applicationId.ToString("x16"), "mods.json");
 
-            if (Application.Current.ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
-            {
-                _storageProvider = desktop.MainWindow.StorageProvider;
-            }
+            _storageProvider = RyujinxApp.MainWindow.StorageProvider;
 
             LoadMods(applicationId);
         }

src/Ryujinx/UI/ViewModels/TitleUpdateViewModel.cs

@@ -76,10 +76,7 @@ namespace Ryujinx.Ava.UI.ViewModels
 
             ApplicationData = applicationData;
 
-            if (Application.Current.ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
-            {
-                StorageProvider = desktop.MainWindow.StorageProvider;
-            }
+            StorageProvider = RyujinxApp.MainWindow.StorageProvider;
 
             LoadUpdates();
         }