There is absolutely no way flutter unseats java/kotlin unless it abandons dart and adopts kotlin as its language. Its much more likely that flutter gets absorbed in some way by the android ecosystem.
Also, I'd be curious to hear what major advantages Kotlin would have for Flutter. The Flutter team has commented heavily on Dart's useful properties for Flutter's goals (small comment here, there are well written full articles as well: flutter.dev/docs/resources/faq#why-did-flutter-choose-to-use-dart).
The internal investment in Flutter is well beyond the average tech project at Google. I don't expect it to be abandoned any time soon.
So if Ads, Pay and Search keep their support, Dart and Flutter are here to stay.
So my read of Flutter is it’s a way for talent to “ship and get promoted” inside Google or to support other Google product teams. Everyone else is a secondary priority.
“the Ads team at Google” is a very strange thing to treat as equivalent to “no one”.