code: void store(String userId, Credential credential) {
      Preconditions.checkNotNull(userId);
      FilePersistedCredential fileCredential = credentials.get(userId);
      if (fileCredential == null) {
        fileCredential = new FilePersistedCredential();
        credentials.put(userId, fileCredential);
      }
      fileCredential.store(credential);
    }
docs: Store information from the credential.
  
  @param userId user ID whose credential needs to be stored
  @param credential credential whose {@link Credential#getAccessToken access token},
  {@link Credential#getRefreshToken refresh token}, and
  {@link Credential#getExpirationTimeMilliseconds expiration time} need to be stored