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

[Zeroupper]

Summary

Building 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 Upgrade all packages and plugins #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