[url_launcher] 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 for slight Kotlin/Java differences.
• Updates tests to use constructors instead of builders, since the Kotlin generator doesn't create builders.

Part of flutter/flutter#158287

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.

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 migrates the Android implementation of the url_launcher plugin from Java to Kotlin using Pigeon v26.3.4. It updates the build configuration to include the Kotlin Gradle plugin, sets the JVM target to 17, and replaces the generated Java message classes with Kotlin equivalents. A review comment was raised regarding the use of Kotlin version 2.3.0 in the build configuration, suggesting a switch to the stable 2.0.0 release.