conversationrepository: migrate account certificate if expired

If an account migration occurs, it can add a new device (and will
change the expiration of the account device). However, by doing so,
the new device certificate will not be expired, causing the account
certificate in the repository to not be updated.
So, validateDevice should also update the parent certificate.

Change-Id: Ifefb9d3ef94e0069347fc109f68da139f39208e4