It's almost cake, but in a cup.
To build:
$ make libs_android_build # or libs_android_download, libs_ios_build, libs_ios_download
$ make prepare_dev # load dev signing key (not required on iOS)
$ make cupcake_android_monero # or cupcake_This project currently only supports Monero, but is very welcoming of other wallets making it's way to the project.
Adding new wallet is as simple as creating new <coin>.dart file in coins directory, and adding it to the coins/list.dart, with proper configuration options set.
Building flavors of the app is as simple as providing --dart-define=COIN_MONERO=false, to disable Monero and doing exactly the same for newly added coin, to enable it.