google-settings.json should be checked after prepare instead of beforeInstall

The check for the google-settings.json is made in the before_plugin_install hook, it can be a bit problematic, when you install a cordova plugin, you might not have populated the www folder. From my own experience, I usually install needed plugins before putting my app code inside www.

Furthermore, my company is selling the same app to many clients (changing the brand logo, etc), so to get the google-settings.json copied, if we don't use a custom hook, we need to uninstall and then reinstall the plugin, that's a real pain.

If this script was run after_prepare, it would be more convenient, I think.