Multiple ApiServiceDescriptors in InstructionRequest leading to unintentional multiple instances of objects in the Java SDK

I noticed that that two separate instances of the ApiServiceDescriptors exist in the instruction request.

This is used as a key to access the BeamFnDataGrpcMultiplexer, and new instances are created if a differing ApiServiceDescriptors is used to access it. 

The InstructionRequest .

I encountered this while trying to add some code in ProcessBundleHandler that needed to access the BeamFnDataGrpcMultiplexer and I used the ProcessBundle root ApiServiceDescriptor.

One usage is in the ProcessBundleDescriptor. The second comes from the RemoteGrpcPort. Which actually comes out of a serialized bytes field. See RemoteGrpcPortWrite::fromPTransform

Imported from Jira [BEAM-5972](https://issues.apache.org/jira/browse/BEAM-5972). Original Jira may contain additional context.
Reported by: ajamato@google.com.