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.
Minor nit, but maybe have a final local variable for getAggregatorExpression() since we call it a number of times.