Based on a handful of https://abseil.io/tips, it's generally advised to
only fully-qualify namespaces when in a `using` statement, or when it's
otherwise required for compilation. In all other cases, the general
recommendation is to not fully-qualify.
This change fixes most `grpc.*` namespace uses. There are potential
challenges in trying to make blanket changes to non-gRPC namespace uses,
such as `::testing`, since there is also a `grpc::testing` namespace.
AJ Heller