INT-2961 Make ARPMH.onInit() Final; Add doInit()

https://jira.springsource.org/browse/INT-2961

onInit() relied on subclasses calling super.onInit() to apply the
advice chain and add the BeanFactory to the MessagingTemplate.

At least one extension (smpp) failed to do this, disabling these features.

Make onInit() final and call doInit() for subclass initialization (no-op in ARPMH).

INT-2961 Polishing

- do not increase visibility of doInit in subclasses.
- remove final modifiers from doInit so user subclasses can participate in initialization if needed.