LPS-45608 Change RSSAction to implement method getRSS(HttpServletRequest...

Change com.liferay.portlet.activities.action.RSSAction to override method protected byte[] getRSS(HttpServletRequest request) of com.liferay.portal.struts.RSSAction so that members activities portlet can take advantage of activities portlet to handle rss request.

I did not keep the old override method serveResource because it is a protected method not public, so it should be safe to remove it.