Converts the Pigeon generation from the older callback system to the new Kotlin `suspend` support. While this is not any simpler while the code is Java, it has several benefits:
- It makes future `pigeon` mass updates much simpler, since this is the new default generation mode.
- It simplifies adoption of the JNI-based Pigeon backend, which always uses the `suspend` style.
- It will simplify the tests when converting them to Kotlin.

Part of https://github.com/flutter/flutter/issues/192417

## Pre-Review Checklist

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