[in_app_purchase] Switch to Kotlin Pigeon (#11608)

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.
- Updates tests to use a fake result value, instead of mocking the Java Pigeon response object.

Most of this conversion was done by Gemini; I gave it my previous "Switch to Kotlin Pigeon" PRs for context, and let it do most of the work. Once it had a passing conversion I did some manual cleanup and warning fixing.

This also converts Translator.java, which does SDK<->Pigeon type conversion, to Kotlin, per discussion in the linked issue:
- I used Android Studio's auto-converter.
- I did some manual cleanup (e.g., fixing nullability where it assumed nullable because the Java wasn't annotated, simplifying some switch return statements).
- I added parameter names to all the constructors, so it's easy to see that the params are being passed correctly in review both now and in future changes.
- I had Gemini do a pass over it to make the Kotlin simpler or more idiomatic. The big improvement was it replacing a bunch of manual list construction with `map`s.
- Removed `currencyCodeFromSymbol`, which has been dead code for years.

These steps are separate commits in case it's useful to review them separately

Fixes https://github.com/flutter/flutter/issues/185545

## Pre-Review Checklist

[^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.