In MediaManager, how to get image's full url with specifed site's domain included?

Question

The reason why I want to change the domain of an image url is because I'm providing an API to a third party. And the regulation is all media url and internal link url, needs to replace the default domain to a specific one.

I know that by setting Options.AlwaysIncludeServerUrl = true, the result of MediaManager.GetMediaUrl() will response an absolute path url with domain.

However the domain I got is not what I want, the image url is something like:

http://{ip_address}/-/media/view-explorer/presets/delux-128x128.jpg

I want it be a specific site hostname, which the site is setted in config file. So I decide to switch siteContext before getMediaUrl()

string GetImageItemUrl(Item item, ID templateId)
{
    string src = string.Empty;
    ImageField imageField = item.Fields[templateId];
    if (imageField != null && imageField.MediaItem != null)
    {
        var site = Sitecore.Configuration.Factory.GetSite("xxxSite");
        using (new SiteContextSwitcher(site))
        {
            var image = new MediaItem(imageField.MediaItem);
            MediaUrlOptions mediaUrlOptions = new MediaUrlOptions();
            mediaUrlOptions.AlwaysIncludeServerUrl = true;
            mediaUrlOptions.AbsolutePath = true;
            src = MediaManager.GetMediaUrl(image, mediaUrlOptions);
        }
    }
    return src;
}

I also patched a config, here is the site section in file App_Config/include/projectname/z.projectname.Dev.Settings.config

<sitecore>
    <sites>
        <site name="xxxSite">
            <patch:attribute name="database">master</patch:attribute>
            <patch:attribute name="hostName">test.xxxxxx.com</patch:attribute>
            <patch:attribute name="targetHostName">test.xxxxxx.com</patch:attribute>
        </site>
    </sites>
</sitecore>

But the result is the same as before. Not the https://test.xxxxxx.com/-/media/view-explorer/presets/delux-128x128.jpg as I expected.

So am I using it the right way?

(Updated: It's 8.2 I'm using)