[google_maps_fluter] Switch to Kotlin Pigeon

[stuartmorgan-g]

Replaces the Java Pigeon generator with the Kotlin Pigeon generator, and adjusts the project accordingly:

• Adds Kotlin build setings to Gradle.
• Updates API signatures and number handling for Kotlin/Java generator differences.
• Adds generic Java/Kotlin compat shim to create Result objects from Java, since those haven't been added to the Pigeon generator yet.
• Updates tests to use constructors instead of builders, since the Kotlin generator doesn't create builders.
  • In two cases where replacing the builder pattern would have required major changes, I instead pulled the builders from the old Java generation into the test files.
• Updates tests to use a Java/Kotlin compat shim to read Kotlin Result values, instead of mocking the Java Pigeon response object.

Part of flutter/flutter#158287

Pre-Review Checklist

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• 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.

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

[gemini-code-assist]

Code Review

This pull request updates the google_maps_flutter_android package to version 2.19.8, primarily migrating the internal implementation to use Kotlin Pigeon. This involves significant changes to the Android codebase, including updating build configurations, replacing Java-based Pigeon generated code with Kotlin, and updating the Convert utility and various controllers to support the new Pigeon-generated types. The review comments suggest managing the Kotlin version via a project-level property or version catalog to avoid drift, and point out that error handling in TileProviderController could be improved to better handle tile fetch failures.