[SPARK-12547][SQL] Tighten scala style checker enforcement for UDF registration

We use scalastyle:off to turn off style checks in certain places where it is not possible to follow the style guide. This is usually ok. However, in udf registration, we disable the checker for a large amount of code simply because some of them exceed 100 char line limit. It is better to just disable the line limit check rather than everything.

In this pull request, I only disabled line length check, and fixed a problem (lack explicit types for public methods).

Author: Reynold Xin <rxin@databricks.com>

Closes #10501 from rxin/SPARK-12547.