Move FieldLayoutEntityFormDisplay::getDefaultRegion() and FieldLayoutEntityViewDisplay::getDefaultRegion() to FieldLayoutEntityDisplayTrait

Problem/Motivation

  public function getDefaultRegion() {
    // This cannot be provided by the trait due to
    // https://bugs.php.net/bug.php?id=71414 which is fixed in PHP 7.0.6.
    return $this->getLayoutDefinition($this->getLayoutId())->getDefaultRegion();
  }

With #3053363: Remove support for PHP 5 in Drupal 8.8 we'll no longer be affect by this bug.

Proposed resolution

Move getDefaultRegion to \Drupal\field_layout\Entity\FieldLayoutEntityDisplayTrait

Remaining tasks

User interface changes

None

API changes

New method on trait - this method is currently implemented in both usages in core.

Data model changes

None

Release notes snippet

N/a