CCE: when running in portlet: Remove Servlet dependencies in FaceletViewDeclarationLanguage.java

In FaceletViewDeclarationLanguage.java: createResponseWriter(), getResponseEncoding(), handleFaceletNotFound(), and sendSourceNotFound() each cast to Servlet object. This causes ClassCastExceptions when run in a portlet environment. Each of these calls/casts can be removed and ExternalContext APIs can be used instead to get/set the needed information from the request or response object.