[MailboxEvent] Simpify constructor of FlagsUpdated

public FlagsUpdated(MailboxSession.SessionId sessionId, User user, MailboxPath path, MailboxId mailboxId, List<MessageUid> uids, List<UpdatedFlags> updatedFlags) {
    super(sessionId, user, path, mailboxId);
    this.uids = ImmutableList.copyOf(uids);
    this.updatedFlags = ImmutableList.copyOf(updatedFlags);
}