Move token storage out of the configuration file.
Move token storage out of the configuration file.

Due to this, the configuration file is now read-only.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=158841877

googleads/googleads-shopping-samples,googleads/googleads-shopping-samples,googleads/googleads-shopping-samples,googleads/googleads-shopping-samples,googleads/googleads-shopping-samples,googleads/googleads-shopping-samples