Use `vxrn` instead of `metro` to build JS bundle during native build process

[zetavg]

May use a react-native.config.js to override the bundle command of React Native CLI, which is called during the native build process. But this is not ideal since it depends on the order in the user's package.json for our react-native.config.js is read and defines the bundle command before react-native.

Ref: https://gist.github.com/zetavg/eacd8f8e6ec46213570ab6d1431672be