Adding default icons to config r?

Depends on [new-icon changes in cordova-cli](https://github.com/mozilla-cordova/cordova-cli/compare/mozilla-dev...new-icon) which actually have to move to the newly created cordova-lib repo. I'm waiting for the repo to be populated in [github](https://github.com/apache/cordova-lib) to move the PR. There is no harm in landing this before though.

Some notes:
- Removed 'Creating Firefox OS project' log because CLI already prints 'Creating firefoxos project...'
- We were copying the `www` folder from `cordova-firefoxos` to `platforms/firefoxos`. We definitely don't want to overwrite all those files in `www`. Turns out it was harmless because it was [being wiped out](https://github.com/mozilla-cordova/cordova-cli/blob/new-icon/src/metadata/firefoxos_parser.js#L142) by CLI's `prepare` before copying over the root `www`. I've added the icons to `platform_www` which are copied into the platform `www` folder by `prepare`. I'll create a new PR to clean up the `www` folder.
- The values we had in defaults.xml were also overwriting the root config values, which we also don't want to do.