code: @Nullable
      private Customer retrieveCustomerWithKey(@NonNull CustomerEphemeralKey key)
              throws StripeException {
          return mApiHandler.retrieveCustomer(key.getCustomerId(), key.getSecret());
      }
docs: Calls the Stripe API (or a test proxy) to fetch a customer. If the provided key is expired,
  this method <b>does not</b> update the key.
  Use {@link #updateCustomer(CustomerEphemeralKey, String)} to validate the key
  before refreshing the customer.
  
  @param key the {@link CustomerEphemeralKey} used for this access
  @return a {@link Customer} if one can be found with this key, or {@code null} if one cannot.