DSL: Use generic `AppListener` from`IntFlowBPP`

https://build.spring.io/browse/INTSAMPLES-NIGHTLY-JOB1-1189

Since `spring-integration-even` is optional dependency, the usage of `ApplicationEventListeningMessageProducer` is wrong.
Changing it to the generic `ApplicationListener` doesn't hurt for any inline components from Flow definition, when there is an interest to handle Events.