[shared_preferences] Switch to Kotlin Pigeon

[stuartmorgan-g]

Replaces the Java Pigeon generator with the Kotlin Pigeon generator, and updates API signatures and number handling for Kotlin/Java generator differences.

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 transitions the internal implementation of the shared_preferences_android package from Java-generated Pigeon code to Kotlin-generated Pigeon code. The LegacySharedPreferencesPlugin has been updated to use primitive types and the new Kotlin companion setup methods. Additionally, the pigeon dependency is bumped to version 26.3.4, and autogenerated files in Dart and Kotlin are updated with improved equality, hashing, and error-handling utilities. I have no feedback to provide.