[SPARK-17494][SQL] changePrecision() on compact decimal should respect rounding mode

## What changes were proposed in this pull request?

Floor()/Ceil() of decimal is implemented using changePrecision() by passing a rounding mode, but the rounding mode is not respected when the decimal is in compact mode (could fit within a Long).

This Update the changePrecision() to respect rounding mode, which could be ROUND_FLOOR, ROUND_CEIL, ROUND_HALF_UP, ROUND_HALF_EVEN.

## How was this patch tested?

Added regression tests.

Author: Davies Liu <davies@databricks.com>

Closes #15154 from davies/decimal_round.