[metrics_center] Add retries to unlock a lock file in case of 504 errors

[drewroengoogle]

• Metrics upload failing because an existing lock file. flutter#120440
• Updates the GcsLock class to retry unlocking the file in case a 504 error occurs
• Updates the GcsLock constructor to require a StorageApi object instead of AuthClient, which allows mocking the object, since the AuthClient object isn't actually being used within the GcsLock class besides to create the StorageApi.

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[keyonghan]

LGTM