UserAccessControlHandler::checkFieldAccess() denies access to mail field without varying by the ‘user’ cache context

Problem/Motivation

Discovered while working on #2819335: Resource (entity) normalization should use partial caching.

\Drupal\user\UserAccessControlHandler::checkFieldAccess() does this:

      case 'preferred_langcode':
      case 'preferred_admin_langcode':
      case 'timezone':
      case 'mail':
        // Allow view access to own mail address and other personalization
        // settings.
        if ($operation == 'view') {
          return $is_own_account ? AccessResult::allowed()->cachePerUser() : AccessResult::neutral();
        }

Note that the "else" case returns AccessResult::neutral() without any cache context. Despite that result being dependent on whether the User entity matching the current user ($is_own_account).

Proposed resolution

Fix it: add the user cache context to the "else" case too.

Remaining tasks

Tests.

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

None.