Drupal 8 custom entity without bundle

example.php

<?php
/**
 * @file
 * Contains \Drupal\example\Entity\Example.
 */

namespace Drupal\example\Entity;

use Drupal\Core\Entity\EntityStorageInterface;
use Drupal\Core\Field\BaseFieldDefinition;
use Drupal\Core\Entity\ContentEntityBase;
use Drupal\Core\Entity\EntityTypeInterface;
use Drupal\example\ExampleInterface;
use Drupal\user\UserInterface;

/**
 * Defines the Example entity.
 *
 * @ingroup example
 *
 * @ContentEntityType(
 *   id = "example",
 *   label = @Translation("Example entity"),
 *   handlers = {
 *     "view_builder" = "Drupal\Core\Entity\EntityViewBuilder",
 *     "views_data" = "Drupal\views\EntityViewsData",
 *     "list_builder" = "Drupal\example\Entity\Controller\ExampleListBuilder",
 *   },
 *   base_table = "examples",
 *   admin_permission = "administer example entity",
 *   fieldable = FALSE,
 *   entity_keys = {
 *     "id" = "id",
 *     "label" = "title",
 *   },
 *   links = {
 *     "collection" = "/example/list"
 *   },
 * )
 */
class Example extends ContentEntityBase implements ExampleInterface {

  /**
   * {@inheritdoc}
   */
  public function getChangedTimeAcrossTranslations()  {
    $changed = $this->getUntranslated()->getChangedTime();
    foreach ($this->getTranslationLanguages(FALSE) as $language)    {
      $translation_changed = $this->getTranslation($language->getId())->getChangedTime();
      $changed = max($translation_changed, $changed);
    }
    return $changed;
  }

  /**
   * {@inheritdoc}
   */
  public function getCreatedTime() {
    return $this->get('created')->value;
  }

  /**
   * {@inheritdoc}
   */
  public function getChangedTime() {
    return $this->get('changed')->value;
  }

  /**
   * {@inheritdoc}
   */
  public function setChangedTime($timestamp) {
    $this->set('changed', $timestamp);
    return $this;
  }

  /**
   * {@inheritdoc}
   *
   * Define the field properties here.
   *
   * Field name, type and size determine the table structure.
   *
   * In addition, we can define how the field and its content can be manipulated
   * in the GUI. The behaviour of the widgets used can be determined here.
   */
  public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {

    // Some example fields.
    $fields['id'] = BaseFieldDefinition::create('integer')
      ->setLabel(t('ID'))
      ->setSettings([
        'size' => 'big',
      ])
      ->setDescription(t('The ID of the Example entity.'))
      ->setReadOnly(TRUE);

    $fields['uid'] = BaseFieldDefinition::create('integer')
      ->setLabel(t('Customer user id'))
      ->setDescription(t('Customer user id in origin'))
      ->setSettings(array(
        'default_value' => null,
      ))
      ->setDisplayOptions('view', array(
        'label' => 'above',
        'type' => 'int',
      ))
      ->setDisplayConfigurable('view', TRUE)
      ->setReadOnly(TRUE);

    $fields['title'] = BaseFieldDefinition::create('string')
      ->setLabel(t('Contact textfield'))
      ->setDescription(t('Example textfield property'))
      ->setSettings(array(
        'default_value' => '',
        'max_length' => 255,
      ))
      ->setDisplayOptions('view', array(
        'label' => 'above',
        'type' => 'string',
      ))
      ->setDisplayConfigurable('view', TRUE);


    //Meta fields
    $fields['created'] = BaseFieldDefinition::create('created')
      ->setLabel(t('Created'))
      ->setDescription(t('The time that the entity was created.'));

    $fields['changed'] = BaseFieldDefinition::create('changed')
      ->setLabel(t('Changed'))
      ->setDescription(t('The time that the entity was last edited.'));

    return $fields;
  }
}