Make use of USINGS

Author: dellis1972

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/PackagingTest.cs

@@ -82,7 +82,7 @@ public void CheckIncludedAssemblies ([Values (false, true)] bool usesAssemblySto
 				Version = "2.14.1",
 			});
 			proj.MainActivity = proj.DefaultMainActivity
-				.Replace ("using System;", @"using System;
+				.Replace ("//${USINGS}", @"using System;
 using Humanizer;
 using System.Globalization;")
 				.Replace ("//${AFTER_ONCREATE}", @"var c = new CultureInfo (""es-ES"");