[SPARK-14774][SQL] Write unscaled values in ColumnVector.putDecimal

## What changes were proposed in this pull request?

We recently made `ColumnarBatch.row` mutable and added a new `ColumnVector.putDecimal` method to support putting `Decimal` values in the `ColumnarBatch`. This unfortunately introduced a bug wherein we were not updating the vector with the proper unscaled values.

## How was this patch tested?

This codepath is hit only when the vectorized aggregate hashmap is enabled. https://github.com/apache/spark/pull/12440 makes sure that a number of regression tests/benchmarks test this bugfix.

Author: Sameer Agarwal <sameer@databricks.com>

Closes #12541 from sameeragarwal/fix-bigdecimal.