[SPARK-24634][SS][FOLLOWUP] Rename the variable from "numLateInputs" to "numRowsDroppedByWatermark"

[HeartSaVioR]

What changes were proposed in this pull request?

This PR renames the variable from "numLateInputs" to "numRowsDroppedByWatermark" so that it becomes self-explanation.

Why are the changes needed?

This is originated from post-review, see #28607 (comment)

Does this PR introduce any user-facing change?

No, as SPARK-24634 is not introduced in any release yet.

How was this patch tested?

Existing UTs.

[HeartSaVioR]

cc. @zsxwing

[SparkQA]

Test build #124013 has finished for PR 28828 at commit cf147aa.

• This patch fails Scala style tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #124017 has finished for PR 28828 at commit a712b4d.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #124015 has finished for PR 28828 at commit ca3b3de.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[HyukjinKwon]

@xuanyuanking too FYI

[xuanyuanking]

LGTM, thanks for the quick follow-up.

[HyukjinKwon]

Looks good to me too. I will merge in few days.

[SparkQA]

Test build #124040 has finished for PR 28828 at commit 75d12d3.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[zsxwing]

Left one minor comment. Otherwise LGTM. Thanks for fixing this!

[SparkQA]

Test build #124073 has finished for PR 28828 at commit 2e984f3.

• This patch fails Scala style tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #124074 has finished for PR 28828 at commit c496eb5.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[HyukjinKwon]

Merged to master.

[HeartSaVioR]

Thanks all for reviewing and merging!