Issue #2689701 by sidharth_k: Significantly simplify logic of Full Page html...

Issue #2689701 by sidharth_k: Significantly simplify logic of Full Page html conversion (no change in functionality)
- Better separation of concerns: Call drupal_deliver_html_page() to handle cases we're not interested in
- Also don't instantiate AMP object if we're not going to be doing any Full HTML conversion