Previously, one SpringApplicationAdminMXBeanRegistrar was created
per context. When there was more then one context this would result
in a javax.management.InstanceAlreadyExistsException being thrown
as an attempt was made to register the MBean more than once.

This commit updates SpringApplicationAdminJmxAutoConfiguration so
that the registrar is only created when there's no such existing bean
in the context hierarchy.

Closes gh-6378