Java README template needs an `api_name` in addition to `name` and `name_pretty`

[`java-pubsublite-kafka`](https://github.com/googleapis/java-pubsublite-kafka/blob/master/.repo-metadata.json) and [`java-pubsublite-spark`](https://github.com/googleapis/java-pubsublite-spark/blob/master/.repo-metadata.json) are libraries tied to the Pub/Sub Lite API, which has its own library [`java-pubsublite`](https://github.com/googleapis/java-pubsublite/blob/master/.repo-metadata.json).

Because the current `.repo-metadata.json` files don't differentiate between service API names and the repo names, the generated README files are misleading at places.

For example, the [paragraph](https://github.com/googleapis/java-pubsublite-spark#getting-started) below should say "Pub/Sub Lite" instead of "Pub/Sub Lite Spark Connector" (which comes from `{{metadata['repo']['name_pretty']}}`).


This could be fixed if an `api_name` exists in `.repo-metadata.json` or if the template derives the API name from the `api_id` field instead of the `name` field.