Clean up type hierarchy of the kernel parser implementations

Minor stuff I want to clean up while implementing the routine new parser work for WFCORE-3309:

1) StandaloneXml/DomainXml/HostXml should not extend CommonXml as they don't use anything from it. They should directly declare the parsing/marshaling interfaces they implement.

2) CommonXml should not declare it implements XMLElementWriter, thus freeing its many subclasses that will never be used in marshaling from needing to add a writeContent method that throws UnsupportedOperationException or does nothing. Only the current version needs to have such a method, and it doesn't need to be public or implement the interface as it's only called by the StandaloneXml/DomainXml/HostXml that's in its package.

3) Various classes can be final, various methods can be private, a few other IDE warns can be cleaned up.