feat: cross platform projects.

demo/Ursa.Demo.Android/MainActivity.cs

@@ -0,0 +1,20 @@
+using Android.App;
+using Android.Content.PM;
+using Avalonia;
+using Avalonia.Android;
+
+namespace Ursa.Demo.Android;
+
+[Activity(
+    Label = "Ursa.Demo.Android",
+    Theme = "@style/MyTheme.NoActionBar",
+    Icon = "@drawable/icon",
+    MainLauncher = true,
+    ConfigurationChanges = ConfigChanges.Orientation | ConfigChanges.ScreenSize | ConfigChanges.UiMode)]
+public class MainActivity : AvaloniaMainActivity<App>
+{
+    protected override AppBuilder CustomizeAppBuilder(AppBuilder builder)
+    {
+        return base.CustomizeAppBuilder(builder);
+    }
+}