"unknown enum constant" warnings when building with Bazel

[drigz]

If I run bazel build //... in grpc-java at HEAD (or v1.16.1) I see many warnings like this:

INFO: From Building core/libinternal.jar (110 source files):
warning: unknown enum constant ReflectionSupport$Level.FULL
  reason: class file for com.google.j2objc.annotations.ReflectionSupport$Level not found

This was discussed on bazelbuild/bazel#6414 and is due to a missing compile-time dependency of guava on j2objc.

What version of gRPC are you using?

HEAD

What did you expect to see?

No warnings when building.

[carl-mastrangelo]

cc: @ericgribkoff