fix: set launchShowDuration to 0 on new projects only

[jcesarmobile]

Instead of setting the default native value to 0 as done on b29346b

set the default values back to 3000 and make new projects have

"plugins": {
    "SplashScreen": {
        "launchShowDuration": 0
    }
}

[imhoffd]

Should it just be showDuration to match the plugin options at runtime?

[jcesarmobile]

It was called launchShowDuration as it's only applied on the launch of the app, while showDuration is for API calls to show method.
We can't change the name now, it's an existing preference.