Use Java list instead of Guava list to avoid null checks with Guava.

There is a bug in Optiq currently that throws errors in guava list because of null entries in given list. The given list is a subset of ```SqlTypeExplicitPrecedenceList.NUMERIC_TYPES``` which has nulls in it. This code is kicked when Optiq trying to prune the list of functions if there are more than one. Because hive udfs are added to operator table, we have few functions defined twice.