[SPARK-18894][SS] Fix event time watermark delay threshold specified in months or years

## What changes were proposed in this pull request?

Two changes
- Fix how delays specified in months and years are translated to milliseconds
- Following up on #16258, not show watermark when there is no watermarking in the query

## How was this patch tested?
Updated and new unit tests

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #16304 from tdas/SPARK-18834-1.

(cherry picked from commit 607a1e63)
Signed-off-by: Shixiong Zhu <shixiong@databricks.com>