The BaseFieldOverride entity fails to normalize default values into the "array keyed by delta" format in the same way BaseFieldDefinition does when a callback is specified

Problem/Motivation

Discovered in test fails in #2949964: Add an EntityOwnerTrait to standardize the base field needed by EntityOwnerInterface.

BaseFieldDefinition::getDefaultValue normalizes default field values into always being an array keyed by delta. FieldConfigBase::getDefaultValue does not do this. When a BaseFieldOverride entity is created default values that used to work no longer do.

Proposed resolution

Fix \Drupal\Core\Field\FieldConfigBase::getDefaultValue to include the same logic as \Drupal\Core\Field\BaseFieldDefinition::getDefaultValue.

Remaining tasks

User interface changes

API changes

Data model changes