Set up Gradle build for yogacore

Summary:
I put the CMake file in the root directory to avoid moving the actual `yoga/`
dir around, but created a directory for the Android library manifest nonsense
so it works as its own module you can depend on.

Reviewed By: emilsjolander

Differential Revision: D4913601

fbshipit-source-id: 582064264cf0b6b69c03d0e72ed9dd22d217dd16

build.gradle

@@ -5,7 +5,7 @@ buildscript {
         jcenter()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:2.2.2'
+        classpath 'com.android.tools.build:gradle:2.3.1'
         classpath 'com.jfrog.bintray.gradle:gradle-bintray-plugin:1.7.3'
         classpath 'com.github.dcendents:android-maven-gradle-plugin:1.4.1'
         classpath 'com.nabilhachicha:android-native-dependencies:0.1'