Given the legal wrangling with Oracle, I don't think they will be eager to support Java. However, Kotlin has a native compiler (llvm based) as well. You can target Android/IOS with it as well as Wasm or simple desktop/server executables. So, the smart move would be to deemphasize Dart, move flutter on top of Kotlin native/llvm (and why not expose it to other languages as well; e.g. swift). Then they cab move the entire Android ecosystem on top of llvm and provide some level of continuity for people with Kotlin/Android codebases.
The main challenge here is the API gap between the old java 6 APIs that underly the current Android API that won't be there on native Kotlin. Flutter solves that problem.
Do you have anything specific in mind about Oracle-Java? They opensource a lot in the last two years, OracleJava is basically 99.9% openJDK now, the main JDK is OpenJDK, Flight Recorder is open-source, graalVM is GPL2...
The problems between oracle and google are related to copyright and APIs. Androids OSS java bits are licensed differently(Apache license if I recall correctly) and technically a partially API compatible implementation of Java about 10 years ago with lots of Google specific additions and some code inherited from IBMs harmony project.
In any case, compiler technology has moved forward a lot and as Apple has been showing with IOS and Swift, you can do some pretty decent app ecosystem around native code without having to expose your developers to doing C/C++.
The main challenge here is the API gap between the old java 6 APIs that underly the current Android API that won't be there on native Kotlin. Flutter solves that problem.