icon/export.sh script is outdated for adaptive icons

[agateau]

Describe the bug

Launcher icons are generated from the icon/export.sh, but this script is outdated for Android icons now that the app uses an adaptive icon (since #83)

How to reproduce

1. Make sure the generate_android_icons() is called in icon/export.sh
2. Run the script
3. It produce files in the wrong asset folders
4. There is no way to regenerate some of the icons (for example https://github.com/agateau/pixelwheels/blob/b063fe65d879e9680bff225ead9ddf3bf67711af/android/res/mipmap-xhdpi/ic_launcher.png)