feat: Offer to run `flutter build apk` for users?

[eseidel]

eseidel@erics-mbp % ../_shorebird/shorebird/bin/shorebird init       
✗ Detecting product flavors (2.1s)
Unable to extract product flavors.
Could not find android/gradlew.
Make sure you have run "flutter build apk" at least once.

[mateusfccp]

Should we do it automatically or should we prompt for user input? Like [Y/n]