[SPARK-19859][SS] The new watermark should override the old one

## What changes were proposed in this pull request?

The new watermark should override the old one. Otherwise, we just pick up the first column which has a watermark, it may be unexpected.

## How was this patch tested?

The new test.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #17199 from zsxwing/SPARK-19859.