function getStatusBarBackgroundColorDir (projectRoot, platformProjDir) {
    if (folderExists(path.join(projectRoot, platformProjDir, 'Assets.xcassets/'))) {
        return path.join(platformProjDir, 'Assets.xcassets', 'StatusBarBackgroundColor.colorset');
    } else {
        return null;
    }
}
Returns the directory for the StatusBarBackgroundColor.colorset asset, or
null if no xcassets exist.

@param  {string} projectRoot        The project's root directory
@param  {string} platformProjDir    The platform's project directory