ModuleApi: Should threepid properties `validate_at` and `added_at` be 0?

[anoadragon453]

This was spun out of #7684

When ModuleApi.register() has emails= set to some emails, those emails are created and added to the new user's account.

However, in the database validated_at and added_at are set to 0, which some other may misinterpret.

Is this intentional or an oversight?

I've at least added the behaviour to a test here: https://github.com/matrix-org/synapse/pull/7684/files#diff-f7ba2d6448431a208d9b515b904ed854R49-R50 Which should be updated as part of closing this issue.