turn Raylib-cs.Android into android library

Raylib-cs.Android/RaylibActivity.cs

@@ -0,0 +1,17 @@
+using System.Runtime.InteropServices;
+
+namespace Raylib_cs;
+
+public abstract class RaylibActivity : NativeActivity
+{
+    protected override void OnCreate(Bundle? savedInstanceState)
+    {
+        base.OnCreate(savedInstanceState);
+        RaylibSetAndroidCallback(OnReady);
+    }
+
+    protected abstract void OnReady();
+
+    [DllImport("raylib")]
+    private static extern void RaylibSetAndroidCallback(Action callback);
+}