docs: fix API key name in digital-payment-credentials README

[dzuluaga]

Summary

• The README instructed users to set GOOGLE_API_KEY in local.properties, but build.gradle.kts reads GEMINI_API_KEY. This mismatch caused a 403 PERMISSION_DENIED error when the Android app called the Gemini API with an empty key.
• Updated the README to use GEMINI_API_KEY for local.properties

Test plan

• Followed the updated README instructions with GEMINI_API_KEY in local.properties
• Verified the Android app successfully calls the Gemini API without 403 errors

Fixes #204

[gemini-code-assist]

Code Review

This pull request updates the README for the digital-payment-credentials scenario to use GEMINI_API_KEY instead of GOOGLE_API_KEY. A review comment points out that the file path in the setup instructions incorrectly references the shopping_assistant project directory instead of the digital-payment-credentials directory.

[gemini-code-assist]

The file path samples/android/shopping_assistant/local.properties seems incorrect as this README.md is located in samples/android/scenarios/digital-payment-credentials/. Assuming the commands are run from the repository root, the path to the shopping_assistant project's local.properties file should be samples/android/shopping_assistant/local.properties.

However, the current path is samples/android/shopping_assistant/local.properties, which seems to be a typo. It should likely be samples/android/shopping_assistant/local.properties to correctly target the shopping_assistant project's properties file from the repository root. This same potential typo exists for the sdk.dir command below.

[dzuluaga]

Lint failures are pre-existing on main (MD030/MD040 in this README). This PR only changes the API key name on lines 50-54.

[dzuluaga]

Re: gemini-code-assist feedback — The path samples/android/shopping_assistant/local.properties is correct. The README instructions are intended to be run from the repository root.

[gemini-code-assist]

Thank you for the clarification, @dzuluaga. That makes sense regarding the intended execution context for the README instructions. I have noted that the lint failures are pre-existing and unrelated to these changes.