Clean up resource generation pipeline

[CedricReichenbach]

By default, Ionic CLI re-generates resources on every platform add. This is problematic because we use a slightly customized setup for android icons (with rounded corners, in a different directory, generated through a manually executed script).

Either prevent resources-generation from running automatically, or somehow register a hook script making sure we end up with rounded corners on Android.

Corresponding SO question: https://stackoverflow.com/questions/50948744/prevent-ionic-cli-from-running-resource-generation-on-platform-add