Add Xamarin Android support

Summary:
Adds a Xamarin Android support for Yoga and respective tests. Closes #276 with this PR all Xamarin platforms will be covered

Need to figure a better strategy for the build names for buck with splhack .

It's failing 3 tests related with YogaNode and the GC :) classic!

![screenshot_1484353863](https://cloud.githubusercontent.com/assets/1235097/21950981/61020212-d9f8-11e6-80e1-185bd1707f9a.png)

```
Facebook.Yoga.Android.Tests.dll : 660.87 ms
 : 678.788 ms
Tests run: 130, Passed: 127, Failed: 3, Skipped: 0, Inconclusive: 0
```
Closes https://github.com/facebook/yoga/pull/340

Reviewed By: emilsjolander

Differential Revision: D4475370

Pulled By: splhack

fbshipit-source-id: f050f10415e68e9808f629b843682b0f87cca065

csharp/Android/Facebook.Yoga.Android.Tests/MainActivity.cs

@@ -0,0 +1,25 @@
+/**
+ * Copyright 2014-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the license found in the
+ * LICENSE-examples file in the root directory of this source tree.
+ */
+
+using System.Reflection;
+using Android.App;
+using Android.OS;
+using Xamarin.Android.NUnitLite;
+
+namespace Facebook.Yoga.Android.Tests
+{
+	[Activity(Label = "Facebook.Yoga.Android.Tests", MainLauncher = true)]
+	public class MainActivity : TestSuiteActivity
+	{
+		protected override void OnCreate(Bundle bundle)
+		{
+			AddTest(Assembly.GetExecutingAssembly());
+			base.OnCreate(bundle);
+		}
+	}
+}