Summary:
This PR make [server port of react-devtools](https://github.com/facebook/react-devtools/blob/master/shells/electron/index.html#L71) can be changed, currently we can set the port yourself and open it, so I think it would be better if it could also be set here.
Another reason is I can debug two RN app (`react-native start --port 8082`), but I cannot let them use react-devtools together, it would be better if I set
different `__REACT_DEVTOOLS_PORT__` in `index.ios.js` and `index.android.js`.
Closes https://github.com/facebook/react-native/pull/10522
Differential Revision: D4144011
Pulled By: hramos
fbshipit-source-id: de67931f377092871a0fe92e7d9a18799625217e