Make GcsUtil use GoogleCloudStorage

GcsUtil is a primary class to access Google Cloud Storage on Apache Beam. Current implementation directly creates GoogleCloudStorageReadChannel and GoogleCloudStorageWriteChannel by itself to read and write GCS data rather than using GoogleCloudStorage which is an abstract class providing basic IO capability which eventually creates channel objects. This request is about updating GcsUtil to use GoogleCloudStorage to create read and write channel, which is expected flexible because it can easily pick up the new change; e.g. new channel implementation using new protocol without code change.