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

## What changes were proposed in this pull request?

A follow up to SPARK-19859:

- extract the calculation of `delayMs` and reuse it.
- update EventTimeWatermarkExec
- use the correct `delayMs` in EventTimeWatermark

## How was this patch tested?

Jenkins.

Author: uncleGen <hustyugm@gmail.com>

Closes #17221 from uncleGen/SPARK-19859.