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/Properties/AndroidManifest.xml

@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="utf-8"?>
+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+    android:versionCode="1"
+    android:versionName="1.0"
+    package="Facebook.Yoga.Android.Tests">
+
+    <uses-sdk
+        android:minSdkVersion="19" />
+
+    <application
+        android:label="Facebook.Yoga.Android.Tests">
+    </application>
+
+</manifest>