I figured out a solution. First of all I didn't want to add that script tag to the gwt.xml file. I didn't realize my app had a main HTML file. I added this to the head tag there.
<script type="text/javascript" src="https://www.google.com/jsapi" ></script>
I could then reference the google object in JNSI via $wnd.google. I was still having trouble with the actual load call though. It was causing the page clear and just hang. I decided to try loading the API with this code that I pulled from VisualationUtils from the GWT wrapper API.
AjaxLoaderOptions options = AjaxLoaderOptions.newInstance();
options.setPackages("corechart");
AjaxLoader.loadApi("visualization", "1", new Runnable() {
@Override
public void run() {
jsniCall();
}
This did the trick. I'd still like to know why I can't load the API in the JSNI method but this works for me.