Update headers

Microsoft.Toolkit.Sample.UWP.App/App.xaml.cs

@@ -1,4 +1,8 @@
-using System;
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
 using Windows.ApplicationModel;
 using Windows.ApplicationModel.Activation;
 using Windows.UI.Xaml;

Microsoft.Toolkit.Sample.UWP.App/MainPage.xaml.cs

@@ -1,4 +1,8 @@
-using System;
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;

Microsoft.Toolkit.Sample.UWP.App/MyUserControl.xaml.cs

@@ -1,4 +1,8 @@
-using System;
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;

Microsoft.Toolkit.Sample.UWP.App/Properties/AssemblyInfo.cs

@@ -1,4 +1,8 @@
-using System.Reflection;
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Reflection;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;