add JavaReadViaImpulse substitution to Java expansion service

Read transform is not supported in the portability framework. `PortableRunner` substitutes Read transform with Impulse + DoFn before it submits a pipeline to JobServer. However, when ExternalTransform is used, this substitution logic doesn't work. Looks like it should be done in the expansion service instead until we completely remove JavaReadViaImpulse hack.