[AGP 9] Restructure addFlutterTasks for separate library and app plugin logic

## Background
AGP 9 transitions exclusively to the new DSL interfaces and hides the legacy variant API. To opt into the new DSL, the Flutter Gradle Plugin should align with the plugin structure that AGP uses. Separating the task addition logic into distinct library and application execution paths will make the rest of the migration easier to apply.

## Work
- [ ] Restructure `addFlutterTasks` to separate the code that runs on libraries from the code that runs on apps.
- [ ] Evaluate keeping a single method that handles shared logic in a "both" section first, followed by the specific library and app sections.
- [ ] Add tests to validate the updated logic.