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.
There was a refactoring in cad09af796 that separated (in the Scala code) the Iface+FutureIface from the FinagledClient, so the FinagledClient was generated into its own file (instead of as an inner class).
So, what used to be just a template for a part of a class (finagleClient.scala and finagleService.scala) is now generating a file of its own, and required new imports, like the ones here:
https://github.com/eirslett/scrooge/commit/cad09af7963f01a3efbb8d5a5add8e7354bda1aa#diff-4

This is the same change, but for Java. Since we generate a new Java file, we need to import all the stuff again.