[scala] Change ScalaAggregateOperator to use TypeSerializer

Before it was using TypeSerializerFactory. This led to problems with the
user code class loader and is not necessary.
At least for now, could we change the signature of the constructor to TypeSerializer and add validate for the argument if it could be cast to TupleSerializerBase?

So add check:
Validate.isTrue(serializer instanceof TupleSerializerBase);

Casting without proper check makes me nervous.