[react-native] Set NODE_ENV and use node-env-inline plugin

Summary:
This sets NODE_ENV based on the value of the `dev` option when bundling the apps. This would then be inlined by the node-env-inline babel plugin. And finally -- if unreachable -- will be dead-code-eliminated by uglify.

This is not used in development because we set NODE_ENV to the value of __DEV__, which can be switched via a query param. However, the plugin has minimal overhead and to avoid complexity in the transformers I just enabled it by default.

packager/transformer.js

@@ -35,6 +35,7 @@ function transform(srcTxt, filename, options) {
       'react',
       'regenerator',
     ],
+    plugins: ['node-env-inline'],
     sourceFileName: filename,
     sourceMaps: false,
     extra: options || {},