Entity attached to "original" property needs to be decrypted in update hooks

Problem/Motivation

If you implement an entity type update hook eg `my_module_node_update()`, you are unable to determine if an encrypted field value has changed by utilizing the `original` property as its fields are still encrypted.

Steps to reproduce

1. Set up a `Test` content type with an encrypted text field `Test`
2. Paste the example node update hook code below into a custom module
3. Create a new `Test` node and then edit it and change the `Test` field value
4. If you debug the node update hook code you will see the `$current_value` is correctly decrypted but the `$previous_value` is still encrypted.

Test code:

/**
 * Implements hook_ENTITY_TYPE_update().
 */
function my_module_node_update(NodeInterface $entity) {
  $current_value = $entity->field_test->getString();
  $previous_value = $entity->original->field_test->getString();
}

Proposed resolution

Modify update hook code to also decrypt the `original` property if it exists.