Migrate DevServerHelper to shared OkHttpClient (#55576)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/55576

Replace the inline OkHttpClient.Builder() in DevServerHelper with DevSupportHttpClient.httpClient. This shares the connection pool and thread pool with all other devsupport HTTP consumers. Remove the now-unused HTTP_CONNECT_TIMEOUT_MS constant and TimeUnit import.

Changelog: [Internal]

Reviewed By: cipolleschi

Differential Revision: D93480251

fbshipit-source-id: b783c2b7a499d20fc12a9fca664651e99b00579d