Prevent retention of reference to child context in parent listener

This was discovered here: https://github.com/spring-projects/spring-xd/issues/661

Sorry for the formatting mess. Just notice I made onApplicationEvent final.
See https://github.com/spring-projects/spring-xd/blob/master/spring-xd-module/src/main/java/org/springframework/xd/module/core/SimpleModule.java#L200