Generate FinagledClient and FinagledService for java-experimental

Support was probably broken in commit cad09af796 when
the *Client and *Service classes were moved out of the "main"
service file. This change was reflected in the generated Scala code,
but not in the Java code.

See also: https://groups.google.com/forum/#!topic/finaglers/W4hhEcAbocI

- Configure compiler to output generated *Client and *Service classes
  to file
- Rewrite relevant Java generators so that they generate code
  compatible with a class living in its own file
  (import statements, remove "static" keyword etc)
- The "main" service class has a FinagledClient which extends
  (...ServiceName...)$FinagleClient, like in the Scala generated code.
Since "The Big Split" aka. cad09af7963, FinagledService and FinagledClient haven't been generated at all, for Java (only for Scala). So the breaking change actually happened in version 3.5.0 (though probably by accident)

It looks like a breaking change at first, but really it isn't - if you look at the [corresponding changes in service.java](https://github.com/eirslett/scrooge/commit/73ad8b6ad615cbb3933468682da5203dd0de8eb7#diff-3). There's now an inner static class names FinagledClient, exactly like it was before. At that class extends this new class. So it's not a breaking change but rather the functionality from version 3.14 is reintroduced. (Since 3.15, the code from the template `finagleClient.java` wasn't being generated at all) I tried to mimic the behavior of the Scala generator as far as I could.