[SPARK-16078][SQL] from_utc_timestamp/to_utc_timestamp should not depends on local timezone

## What changes were proposed in this pull request?

Currently, we use local timezone to parse or format a timestamp (TimestampType), then use Long as the microseconds since epoch UTC.

In from_utc_timestamp() and to_utc_timestamp(), we did not consider the local timezone, they could return different results with different local timezone.

This PR will do the conversion based on human time (in local timezone), it should return same result in whatever timezone. But because the mapping from absolute timestamp to human time is not exactly one-to-one mapping, it will still return wrong result in some timezone (also in the begging or ending of DST).

This PR is kind of the best effort fix. In long term, we should make the TimestampType be timezone aware to fix this totally.

## How was this patch tested?

Tested these function in all timezone.

Author: Davies Liu <davies@databricks.com>

Closes #13784 from davies/convert_tz.