Update to Android Gradle Plugin 9.0.1

[inktomi]

• Accepted Android Studio's migration to add Gradle Toolchain resolution to auto install the JDK when needed, this is the foojay convention plugin you see.
• Migrate Fladle from AGP 7.4.2 to AGP 9.0.1, fixing "Could not find AppExtension error when newDsl=true" (this is the default in AGP 9.0 so we needed to do some refactoring to solve it)
• Update Gradle wrapper from 8.14.3 to 9.1.0 (minimum required by AGP 9.0, not latest Gradle though..)
• Replace legacy variant API (AppExtension, TestedExtension, BaseVariant) with new variant API (ApplicationExtension,
  ApplicationAndroidComponentsExtension, Variant)
• Remove kotlin-android plugin from sample projects (they use the built-in Kotlin in AGP 9.0)
• Ran the formating tools in the project

Closes #478

Tests run locally:

• ./gradlew :fladle-plugin:check — 137 tests pass, lint clean
• ./gradlew assembleDebug assembleDebugAndroidTest printYml — sample projects build with correct YAML output
• ./gradlew :sample-flavors-kotlin:printYml — flavor-specific APK paths resolve correctly
• ./gradlew printYml --configuration-cache — configuration cache works

[inktomi]

The workflow passes on my local fork: https://github.com/inktomi/fladle/actions/runs/22598741034/job/65475320428

[inktomi]

As part two of this, I'm working on reworking the fulladle plugin to support config caching. I will open that PR once this is reviewed, or ideally approved - I didn't want to work on it using old gradle APIs. I am introducing a new Settings plugin that people will have to apply so that we can get information about the modules without breaking caches - we can discuss more on that PR when the time comes.