fix(android): conditionally apply kotlin-android for AGP 9 built-in Kotlin support

[yuki-sakakibara]

AGP 9+ has built-in Kotlin support, and explicitly applying kotlin-android conflicts with it.

This change conditionally applies kotlin-android and kotlinOptions only when AGP < 9.

References

• https://docs.flutter.dev/release/breaking-changes/migrate-to-agp-9
• fix(android): remove kotlin-android since AGP 9 supports it firebase/flutterfire#18059

[onevcat]

Thanks for the PR! The approach looks correct to me — conditionally applying kotlin-android based on the AGP version makes sense.

I'm not super confident with the Android build system though, so @YkSix could you take a look? It would be great if you could also verify this with an AGP 9 environment to make sure everything builds cleanly.

[onevcat]

Since our Android expert is currently on vacation, I verified locally that this change doesn't break existing builds on AGP < 9. Flutter itself doesn't support AGP 9 for plugins yet (see flutter/flutter#181383), so we can't fully verify the AGP 9 path at this time. Merging for now — we'll revisit and do a full verification once Flutter officially supports AGP 9.