Block add form ajax callback implementation issues

Problem/Motivation

The block add route can be accessed without the theme parameter. The theme select will appear at that point. Whenever the theme is changed, an AJAX callback will update the available regions. If a region is picked and then the theme is changed, a validation error will appear if the previously selected region doesn't exists for the new theme.
This is caused by the region list being updated inside the AJAX callback. This also causes the to lose the empty option added in the select element process.
Furthermore, due to this approach, the region list is not alterable during hook_form_alter() implementations.
For example, the block_region_permissions module cannot access the rebuilt list.

Steps to reproduce

How to reproduce the validation error:

1. Login as a user with the administer blocks permission and visit admin/structure/block/add/system_powered_by_block (any other block will be fine);
2. Select the Seven theme, then select the "Pre-content" region;
3. Select back the Bartik theme. The error will be shown. Also notice the missing empty option.

Proposed resolution

Build the correct region list in the form, and just return the form element in the AJAX callback.

Remaining tasks

Finalise the test coverage.

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

TBD.