[sdk-platform-java] Mock tests are generated for mixin APIs when they probably shouldn't be

I was just testing a change to `build_gen` to introduce the Location pkg deps when the mixin is present and noticed in the generated output that `MockLocations.java` and `MockLocationsImpl.java`. I'm not sure that these should be generated since there isn't a Locations client generated, but I will let you @miraleung decide.