Checkbox billing=shipping address should be (un)checked by customer only and not by M2

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### Preconditions
1. M2.2.5

### Steps to reproduce
1. Put item in cart
2. Go to checkout
3. Enter shipping address
4. Press Next to go to checkout step 2
5. Return to shipping address
6. Correct (edit) the zipcode or whatever other item
7. Press Next to go to checkout step 2

### Expected result
1. Billing address is still shipping address and checkbox is still checked
2. When unchecking the checkbox the shipping address is copied to the billing address ready for editing

### Actual result
1. The billing address is equal to the original shipping address and the checkbox is unchecked (that is if https://github.com/magento/magento2/issues/17925 is implemented like it was told in 2-2-dev)
2. Editing the billing address shows empty fields which is kind of frustrating

As long as the checkbox (Billing=Shipping) is checked and not unchecked by the user, updating the shipping address SHOULD ALSO update the billing address accordingly. This is a far more natural approach (and was implemented as such in Magento1) then to let Magento clear the checkbox ( .... unseen by the user).

Example: Customers make mistakes. A customer discovers in checkout step 2 that there was an error in the shipping (=billing) address. He gets back to Step 1 to correct the error but is unaware that the billing address is not changed accordingly.

Start at the checkout shipping-address flow described in the reproduction steps and trace how returning to edit the shipping address affects the Billing=Shipping checkbox and billing fields. Verify the behavior by repeating the listed steps; done means the checkbox changes only when the customer changes it, and unchecking it leaves the shipping address copied into billing for editing.

Written by the indexing model from the issue text.