Use `flutter` repo for engine golds instead of `flutter-engine`. (#160556)

Closes https://github.com/flutter/flutter/issues/157206.

I also added a `prefix` that will default to `engine.` to avoid
accidentally stomping on golden names across repos.

/cc @gaaclarke for visibility, @Piinks for visibility.

(I would love to get rid of this "engine copy" of the client as part of
longer-term mono repo deduplication).