Make it easier to share default themes between attractor-egui and attractor-android

[Rodrigodd]

The default themes for attractor-egui are stored in default.json, while the ones in the Android app are stored in the Kotlin source code.

It would be easier to maintain if the themes were stored in a single place. This could be done by shipping default.json with the Android app and making the Kotlin code read from it, or by creating a code generator that generates the themes in Kotlin from the JSON file.