Merge FieldItem::insert() and FieldItem::update() on fields into preSave()

FieldItem::insert() and FieldItem::update() are left-overs of the previous field.module API, but do not make much sense in the entity context as insert/update are not post-storage operation as they are usually for entities, but pre-save operations as well.

Let's fix that by making use of $entity->isNew() as in the other entity preSave() method callbacks also.

Postponed on #1969728: Implement Field API "field types" as TypedData Plugins.