flutter-fdroid script

One of the recurring issues with Flutter apps is that we have to run the exact version of Flutter needed by the app (and a lot of apps don't specify it in pubspec.yaml or in the README, so we have to guess).

As discussed in #554 (comment 515792753), this also makes auto-update complicated because we currently hardcode the Flutter version in the srclibs.

So this MR adds a script similar to gradlew-fdroid that detects and runs the correct Flutter version. However, it requires that upstream declares the version in pubspec.yaml (but we could add this as a requirement in RFPs).

I used yq to parse the YAML, but we might not want to add a dependency on this tool.