RFR(s): 8203670: unmodifiable List iterator() implementations should not be ListIterators

Hi all, Please review this small changeset that ensures that calling iterator() on an unmodifiable List (from List.of, et. al.) returns an instance that cannot be downcast to ListIterator and used as such. Bug: https://bugs.openjdk.java.net/browse/JDK-8203670 Webrev: http://cr.openjdk.java.net/~smarks/reviews/8203670/webrev.0/ Thanks, s'marks