Detect google service account file in the project dir

[dsokal]

This is a follow-up to expo/expo-cli#2069.

A comment from @brentvatne:

just an idea - an alternative might be to look in cwd for something that matches this format, or is a json file and contains "type": "service_account", and suggest that one, then only use this value as a fallback if we can't find anything.

another option (and could be done in addition to the above) would be that after the first submission of the app we save away the most recently used service account location to .expo and populate the default with that. but this may be overkill for a small thing like this, and it doesn't really help as much with the first submission

I think that I prefer the first option - we should try to find the service account file and suggest the path in the prompt.