cordova.js needs to be copied to platform www

The [new icon PR](https://github.com/apache/cordova-firefoxos/pull/12) introduced icon files inside the `platform_www` folder. Because `platform_www` folder now exists, `cordova.js` file is not copied over by prepare, [see the code](https://github.com/apache/cordova-lib/blob/master/cordova-lib/src/cordova/prepare.js#L91-L96). We have to manually copy it over.

**Fun Fact**: The [cleanup PR](https://github.com/apache/cordova-firefoxos/pull/11) removed a line that copied the `cordova.js` file to `www` which actually had no effect since the `www` folder was later [wiped out](https://github.com/apache/cordova-lib/blob/master/cordova-lib/src/cordova/metadata/firefoxos_parser.js#L173-L174). This fix just copies the file over to `platform_www` instead of `www`.