@Override
  public InMemAccountService getAccountService() {
    return returnOnlyUnknown ? ONLY_UNKNOWN : notifyConsumers ? ANY_ACCOUNT_NOTIFY : ANY_ACCOUNT_NO_NOTIFY;
  }
Constructor. Each different configuration for these parameters has a singleton {@link AccountService}.
@param returnOnlyUnknown on {@link #getAccountService()}, returns an {@link AccountService} that will only return
                         {@link InMemAccountService#UNKNOWN_ACCOUNT}.

@param notifyConsumers if {@code true}, will notify consumers when accounts are updated. This cannot be
                       {@code true} if {@code returnOnlyUnknown} is {@code true}.
ambry-test-utils/src/main/java/com/github/ambry/account/InMemAccountServiceFactory.java