Include ReactAndroid with the npm distribution

Summary: Enables building RN Android from source while keeping it in sync with the iOS distribution.
Closes https://github.com/facebook/react-native/pull/3019

Reviewed By: @​svcscm

Differential Revision: D2506390

Pulled By: @vjeux

package.json

@@ -25,6 +25,7 @@
   "files": [
     "React",
     "React.podspec",
+    "ReactAndroid",
     "Libraries",
     "packager",
     "cli.js",