[in_app_purchase_android] Add obfuscated profile ID purchase param

[lianshumin]

Adds an optional obfuscatedProfileId parameter to GooglePlayPurchaseParam
and passes it through to Google Play Billing's
BillingFlowParams.Builder.setObfuscatedProfileId().

This keeps the API Android-specific, since setObfuscatedProfileId() is a
Google Play Billing API and has no StoreKit equivalent. Existing behavior is
unchanged because the new parameter is optional.

Fixes flutter/flutter#130128

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

Note: The Flutter team is currently trialing the use of Gemini Code Assist for GitHub. Comments from the gemini-code-assist bot should not be taken as authoritative feedback from the Flutter team. If you find its comments useful you can update your code accordingly, but if you are unsure or disagree with the feedback, please feel free to wait for a Flutter team member's review for guidance on which automated comments should be addressed.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩²

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[gemini-code-assist]

Code Review

This pull request adds support for passing an obfuscated profile ID (obfuscatedProfileId) in GooglePlayPurchaseParam for the Android in-app purchase implementation, propagating it to the billing flow parameters, and adding a corresponding unit test. The review feedback suggests documenting that obfuscatedProfileId requires applicationUserName to be specified, and updating the unit test to include both parameters for a more realistic scenario.