Flutter is just a UI framework, it still depends on the Android SDK.

I’ve also just started learning Android dev, avoiding Android Studio. So far I’ve followed this blog and have a Java “Hello World” app building out of a makefile (without even Gradle!), but using the Google SDK and build tools.

I believe the build tools are FOSS, but I didn’t know that about the SDK; just searching now Replicant has some links to FOSS SDK-related things but that page hasn’t been updated in 2 years and nothing seems very easy. Would be interested to hear if you manage to get a FOSS version working.