DERBY-5840: Clean up compiler warnings introduced by using Java 5 language features

Fix unchecked warning in GroupedAggregateResultSet by changing the type
of the distinctValues field from HashSet[][] to List<List<Set<String>>>.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1486028 13f79535-47bb-0310-9956-ffa450edef68