DSL: Register `AELMP` as `AL` in the `AEM`

Register `ApplicationEventListeningMessageProducer` as `ApplicationListener` in the `ApplicationEventMulticaster`

Since `ApplicationContext` invokes its `registerListeners()` before the `IntegrationFlowBeanPostProcessor` does its stuff,
the `ApplicationEventListeningMessageProducer` hasn't been visible as an `ApplicationListener` bean, when it is defined from `IntegrationFlow`,
hence add a logic to register it to the `ApplicationEventMulticaster` from `IntegrationFlowBeanPostProcessor`.