fix the initialContent is always null (#15650)

src/Android/Avalonia.Android/AvaloniaMainActivity.cs

@@ -31,6 +31,8 @@ public class AvaloniaMainActivity : AvaloniaActivity
         // We need this AfterSetup callback to match iOS/Browser behavior and ensure that view/toplevel is available in custom AfterSetup calls.
         if (Lifetime is not null)
         {
+            initialContent ??= Lifetime.MainView; 
+
             Lifetime.Activity = this;
             _view = new AvaloniaView(this) { Content = initialContent };
         }