INT-2961 Make ARPMH.onInit() Final

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

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

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