PHOENIX-1362 Fix Min/MaxAggregateFunction Aggregator max length

Turned out the expression compiler was seeing the max length of 0 on fixed length data types because getMaxLength() on the aggregator was not being overridden.

Let me know if I need to change anything especially on the test side.  Not too familiar yet with the style of the project.
By convention, we use test in the test name. How about something like testMinMaxForFixedLength()?