in_app_purchases codelab update

[RobertOdrowaz]

This PR is for a matching docs update. There are only minor changes in the code:

1. googlePlayProjectName -> googleCloudProjectId const rename to make it more appropriate. It is used for Pub/Sub to which Google Play publishes purchase-related events but it's on Google Cloud so we need the ID of the GCP project.
2. Steps 09 and 10 were misaligned relative to the instructions and should be steps 10 and 11 instead.
3. Minor update to backend readme. The constants.dart already exists, there was a typo in one of the placeholder values, and constants.dart uses single quotes.

Pre-launch Checklist

• I read the Effective Dart: Style recently, and have followed its advice.
• I signed the CLA.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.

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