Exposed ability for implmentations of Animation.java to check if the animation is currently running.

Review at http://gwt-code-reviews.appspot.com/1891804

Review by: mdempsky@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11538 8db76d5a-ed1c-0410-87a9-c151d255dfc7