Create variant of google-cloud-java with Guava and grpc-stub shaded

[garrettjonesgoogle]

Note: GAX needs to be exposed.

Consequences:

1. Guava can't be exposed on the public surface of GAX or google-cloud-java
2. grpc-stub can't be exposed on the public surface of GAX or google-cloud-java (since it exposes Guava)
3. The generated grpc packages need to be split into two: a package with proto-generated types (no Guava dependency) and a package with only grpc (which exposes Guava), and the second type of package needs to be shaded.

Coordinating issue in GAX is filed at googleapis/gax-java#188 .

Also: protobuf-java-util needs to be shaded.

@VisibleForTesting needs to be removed throughout the code.

[garrettjonesgoogle]

blocking issues:

• https://github.com/googleapis/api-common-java/issues/8 "Remove Guava from surface (remove from public classes and methods)"
• delete PathTemplate & related classes, depend on api-common-java gax-java#240 "delete PathTemplate & related classes, depend on api-common-java"

[garrettjonesgoogle]

I will migrate Java grpc package generation to toolkit as part of this change.

[vkedia]

We will also need to shade protobuf-java-util and guava that it depends on once PR #1770 is merged.