Migrate audio_streamer/light/screen_state/pedometer to Built-in Kotlin; bump audio_streamer to Kotlin 2.2.20 (Flutter 3.44)

## Summary

Building [`carp-dk/carp_study_app`](https://github.com/carp-dk/carp_study_app) against **Flutter 3.44.0** produces deprecation warnings for several plugins in this repo. Flutter 3.44's `DependencyVersionChecker` now warns below these minimums:

| Dependency | Warn-below threshold (Flutter 3.44) |
|---|---|
| Kotlin (KGP) | **2.2.20** |
| Android Gradle Plugin | **8.11.1** |
| Gradle | **8.14.0** |

In addition, Flutter now warns that **any plugin applying the Kotlin Gradle Plugin the legacy way** (`apply plugin: 'kotlin-android'` + a `buildscript` classpath) will fail to build in a future Flutter release, and must migrate to **Built-in Kotlin**:

> WARNING: Your app uses the following plugins that apply Kotlin Gradle Plugin (KGP): … `audio_streamer`, `light`, `pedometer`, `screen_state` …
> Future versions of Flutter will fail to build if your app uses plugins that apply KGP.

Migration guide: https://docs.flutter.dev/release/breaking-changes/migrate-to-built-in-kotlin/for-plugin-authors

## Current state (master)

| Plugin | `ext.kotlin_version` | AGP classpath | Applies KGP legacy? | Below 3.44 Kotlin min (2.2.20)? |
|---|---|---|---|---|
| `audio_streamer` | **2.2.10** | 8.12.1 | yes | **yes — needs bump** |
| `light` | 2.2.20 | 8.12.1 | yes | no |
| `screen_state` | 2.2.20 | 8.12.1 | yes | no |
| `pedometer` | 2.2.20 | 8.12.1 | yes | no |

## Requested changes

1. **`audio_streamer`** — bump `ext.kotlin_version` from `2.2.10` to **≥ 2.2.20** (it is still unchecked in the tracking list of #1266).
2. **All four plugins** — migrate from `apply plugin: 'kotlin-android'` to **Built-in Kotlin** so they no longer apply KGP directly. This is the only thing that clears the "plugins that apply KGP" warning; bumping the Kotlin version alone does not.

## Context

This extends #1266, whose checklist targets the older `compileSDK 36` / Kotlin 2.1.0 bar. Flutter 3.44 raises the bar to Kotlin 2.2.20 / AGP 8.11.1 / Gradle 8.14.0 and additionally requires the Built-in Kotlin migration, which is not covered there.

Refs #1266