Resolves concerns raised in ENG-2734 and ENG-2707.

This diff introduces two environment variables, whose are read by Expo start command (yarn dev):

• COMM_DEV - This one must be set, otherwise natDevHostname will be null. Its purpose is not to embed developer machine IP in production builds.
• COMM_NAT_DEV_HOSTNAME - This one can be used to overwrite the autodetected IP address - I consider this as a replacement for network.json in special cases we want to have full control over the hostname.

Depends on D6267

The expo config command below evaluates the app.config.js (and some other Expo defaults), it's used internally when running Metro dev server.

cd native

# should be empty
yarn expo config | grep natDevHostname

# should detect local IP
COMM_DEV=1 yarn expo config | grep natDevHostname

# should resolve to '1.2.3.4'
COMM_DEV=1 COMM_NAT_DEV_HOSTNAME=1.2.3.4 yarn expo config | grep natDevHostname