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.
Another important question is - why don't we move this generator back so it's once again generated in the same file as iface and FutureIface - the reason why is explained in the "Big Split" commit. One could say we should only do that in Scala, not in Java, but the common logic between Scala and Java generators "forces" us to separate the files.