[native] Allow getDevServerHostname to be called from release mode

Summary: We don't actually have any code that does this, but I was debugging a release build and wanted to use Reactrotron. Uncommenting the release condition around native/reactrotron.js led to this invariant firing, which I thought was kinda unnecessary.

Test Plan: Confirm the invariant no longer fires after uncommenting the release condition around native/reactotron.js

Reviewers: tomek, atul

Reviewed By: atul

Differential Revision: https://phab.comm.dev/D5692