Summary:
Implemented automatic IP detection for iOS, based on #6345 and #6362.
As the previous pull requests did, this works by writing the IP address of the host to a file.
Closes https://github.com/facebook/react-native/pull/8091
Differential Revision: D3427657
Pulled By: javache
fbshipit-source-id: 3f534c9b32c4d6fb9615fc2e2c3c3aef421454c5