Fix account URI when updating ActivityPub account (#7488)

Updates account `uri` field on each call to `update_account` instead of
only once during `create_account` to mirror the same behavior in OStatus
`ResolveAccountService` class [0].

ActivityPub accounts are identified using `@username` and `@domain` pair
instead of URI since #6842.

This fixes #7479: a bug when the account identified by `@username` and
`@domain` changes its URI.

[0]:
https://github.com/tootsuite/mastodon/blob/03b69ebc450efc07246bd64204276941b7ede3fc/app/services/resolve_account_service.rb#L121