turn Raylib-cs.Android into android library

Raylib-cs.Android.Sample/MainActivity.cs

@@ -0,0 +1,34 @@
+using System.Runtime.InteropServices;
+using Android.Content;
+using Android.Content.PM;
+
+namespace Raylib_cs.Android;
+
+[
+    Activity(
+        Label = "@string/app_name",
+        MainLauncher = true,
+        ConfigurationChanges = ConfigChanges.Orientation
+            | ConfigChanges.KeyboardHidden
+            | ConfigChanges.ScreenSize,
+        ScreenOrientation = ScreenOrientation.Landscape,
+        ClearTaskOnLaunch = true
+    ),
+    IntentFilter(new[] { Intent.ActionMain }),
+    MetaData(NativeActivity.MetaDataLibName, Value = "raylib")
+]
+public class MainActivity : RaylibActivity
+{
+    protected override void OnReady()
+    {
+        Raylib.InitWindow(0, 0, "android_window");
+        while (!Raylib.WindowShouldClose())
+        {
+            Raylib.BeginDrawing();
+            Raylib.ClearBackground(Color.WHITE);
+            Raylib.DrawFPS(10, 10);
+            Raylib.DrawText("Hello Raylib-cs.Android", 20, 100, 100, Color.BLACK);
+            Raylib.EndDrawing();
+        }
+    }
+}