Feature Request: Contact Form: Use email from the account - Google Signup

### What
When I create an account using my email address and then enter a contact form in a post/page, the contact form automatically uses the email address from my account.

However, when I create an account using my Google account, and then add a contact form, the contact form doesn't seem to use the email address from the Google account.

### How
I think we should try to use the email from the Google account in the contact form block.

No files, tests, or entry points are identified. Start by tracing how the contact form block chooses its email and how Google-signup account data is stored, then determine whether that address can be used safely; done means Google-created accounts populate the contact form email as described.

Written by the indexing model from the issue text.