Use ArrayList instead of immutable singleton List

Cannot add additional elements to immutable list in case they need to
reprocessed, see:

https://github.com/vinted/camus/blob/3abb534188aa92cc1974b03546f521459cd724f9/camus-sweeper/src/main/java/com/linkedin/camus/sweeper/CamusSweeperHiveDatePartitionPlanner.java#L70