Prevent usage of default keys for production apps

### Problem description
Currently it is possible to build a Tabris.js Android app in release mode with one of the keys generated by the Tabris.js build service on tabrisjs.com (_Default Key (app-name)_).

It is not recommended to upload such a build to the Play Store:

* for security reasons, on Android separate keys should be used when signing builds for development and production
* the Tabris.js build service does not provide a way to download those keys, so signing a version of the app built on the user's machine with the same key is currently not possible.

### Expected behavior

Default keys should not be listed in the _Release Signing Key_ dropdown box for Android builds.