INT-3404: Remove `MProducer` from `AIFileSMS`

JIRA: https://jira.spring.io/browse/INT-3404

Previously, `AbstractInboundFileSynchronizingMessageSource` implemented both `MessageProducer` and `MessageSource`.
The `MessageProducer` is really redundant and confusing.
Also, it might produce some side-effects when using JavaConfig.

Provide some other simple but important fix for all `AbstractMessageSource` implementation to invoke `super.afterPropertiesSet()`

Polishing

Add mock bean factory to failing tests.

Revert state assertions in AbstractInboundFileSynchronizingMessageSource;
the fields are state, not arguments, at the time they are being
asserted.