Profile fields not updated when toggling between "incomplete" existing profiles

Problem/Motivation

A bit like #3084273: Profile fields not cleared when toggling between "Edit" and "Enter a new address" in that it's also about that same form and updating form values.
But with a different use case:

1. assuming customer has at least 1 profile in address book
2. assuming those profiles incomplete because they eg. were imported via a migration that (by default) lacks some validation
3. on checkout customer selects a different profile than the one that's selected by default
4. AJAX is running and nothing happens. The fields of the profile form are still populated with the data that was there before we selected a different profile

Proposed resolution

Always update form field values after selecting a profile from the address book.

Remaining tasks

Write a patch.

User interface changes

No extra form fields or labels or such. Only the expected values in the form fields.

API changes

None.

Data model changes

None.

Release notes snippet

Switching between incomplete profiles, at the billing information form, now shows the selected profile instead of sticking with the default profile or empty fields after having chosen "Enter a new address".