[SPARK-22130][CORE] UTF8String.trim() scans " " twice

## What changes were proposed in this pull request?

This PR allows us to scan a string including only white space (e.g. `"     "`) once while the current implementation scans twice (right to left, and then left to right).

## How was this patch tested?

Existing test suites

Author: Kazuaki Ishizaki <ishizaki@jp.ibm.com>

Closes #19355 from kiszk/SPARK-22130.