gRPC Java Stubs modification

[Carl Mastrangelo]

If you don't use gRPC Java Stubs you can stop reading now.

In https://github.com/grpc/grpc-java/pull/4506   I will be removing the static fields that store the MethodDescriptors.   The reason for the change is that it allows lazy loading of the Protobuf classes, which may be very large.   This should make Android binaries able to remove references to unused methods and shrink in size.

The field names look like METHOD_FOO  and have been deprecated and experimental for a while now.  I have tried to find all the known usages of these fields, but as they are generated it's hard to find all references.  Please let me know asap if you are unable use the getter methods (e.g. getFooMethod() ) instead.   Otherwise, they are on track for removal in gRPC 1.13