D7 views variables not converted to Backdrop config

[jenlampton]

Description of the bug

This is a follow-up to #1818.

It was mentioned in the above issue that the variable field_rewrite_elements was not properly converted to config as part of the upgrade from Drupal 7 to Backdrop. We should check other views variables to see if they were converted, and if this is the only remaining variable, add an update hook to pull the overridden value out of the variables table for sites that had set it in D7.

Steps To Reproduce

To reproduce the behavior:

1. In Drupal 7, write a custom update handler to change the value of field_rewrite_elements
2. Upgrade the site to Backdrop.
3. Check to see if the value has been converted.

Actual behavior

Value not saved.

Expected behavior

Value should be retained through upgrade.