UniquePersonList: Simplify logic of updatePerson(...)

[Zhiyuan-Amos]

This method retrieves the person in the list to be updated, and calls
Person#resetData(ReadOnlyPerson) to update the values of that person.

We do not need to retrieve the person in the list to be updated, to
update the list accordingly.

Let’s simplify the logic by setting the updated Person object in place
of the person to be updated.