VaultPress: Update VP when Blog owner changes

### Impacted plugin

VaultPress

### Steps to Reproduce

1. Change the blog owner for a WoA site.
2. Go to VP and see that the owner has not changed.

### A clear and concise description of what you expected to happen.

When a WoA site owner is changed in Blog RC, this should also change the owner in VP for that site.

### What actually happened

At this moment, the old user will still be the only and primary account in VP.

### Other information

More information can be found at:

* 762-gh-Automattic/atomic
* D24708-code
* D24709-code
* CB7AEN79D/p1550790077078200-slack-jetpack-yamato

### Operating System

_No response_

### OS Version

_No response_

### Browser

Other / Not applicable

### Browser Version(s)

_No response_

Start by tracing the owner-change flow from Blog RC for a WoA site into the VaultPress integration, using the linked Atomic and code references as context. Reproduce the change and verify that the new owner becomes the sole primary account in VP instead of the previous owner.

Written by the indexing model from the issue text.