[SPARK-20289][SQL] Use StaticInvoke to box primitive types

## What changes were proposed in this pull request?
Dataset typed API currently uses NewInstance to box primitive types (i.e. calling the constructor). Instead, it'd be slightly more idiomatic in Java to use PrimitiveType.valueOf, which can be invoked using StaticInvoke expression.

## How was this patch tested?
The change should be covered by existing tests for Dataset encoders.

Author: Reynold Xin <rxin@databricks.com>

Closes #17604 from rxin/SPARK-20289.