Replace deprecated entityManager with entityFieldManager

Problem/Motivation

Static code analysis returns the following error:

 ------ --------------------------------------------------------------------------
  Line   office_hours.install
 ------ --------------------------------------------------------------------------
  45     Call to deprecated method entityManager() of class Drupal:
         in Drupal 8.0.0 and will be removed before Drupal 9.0.0.
         Use \Drupal::entityTypeManager() instead in most cases. If the needed
         method is not on \Drupal\Core\Entity\EntityTypeManagerInterface, see the
         deprecated \Drupal\Core\Entity\EntityManager to find the
         correct interface or service.
 ------ --------------------------------------------------------------------------

Proposed resolution

Replace calls to deprecated entityManager with the entityFieldManager.

Remaining tasks

1. Write a patch
2. Review
3. Commit

User interface changes

None.

API changes

None.

Data model changes

None.