Android:

1.- En el archivo styles.xml en Resources/values agregar el siguiente script 

<style name=”Theme.Splash” parent=”android:Theme”>

        <item name=”android:windowBackground”>@drawable/splash</item>

        <item name=”android:windowNoTitle”>true</item>

 </style>

2.- Agregar un Activiy llamado SplashActivity:

using Android.App;
using Android.Content;
using Android.OS;
using System;
using System.Threading.Tasks;

namespace SoyAndi.Droid
{


	[Activity(Theme = "@style/Theme.Splash",
		MainLauncher = true,
		NoHistory = true,
		Icon = "@drawable/icono")]
	public class SplashActivity : Activity
	{
		protected override void OnCreate(Bundle savedInstanceState)
		{
			base.OnCreate(savedInstanceState);

		}

		protected override void OnResume()
		{
			base.OnResume();
			Task startupwork = new Task(() => { SimulateStartup(); });
			startupwork.Start();
		}

		private async void SimulateStartup()
		{
			await Task.Delay(2000);
			StartActivity(new Intent(Application.Context, typeof(MainActivity)));
		}
	}
}

3.- En el MainActivity poner el MainLauncher = false