[SPARK-14625] TaskUIData and ExecutorUIData shouldn't be case classes

[rxin]

What changes were proposed in this pull request?

I was trying to understand the accumulator and metrics update source code and these two classes don't really need to be case classes. It would also be more consistent with other UI classes if they are not case classes. This is part of my bigger effort to simplify accumulators and task metrics.

How was this patch tested?

This is a straightforward refactoring without behavior change.

[rxin]

cc @cloud-fan for review

[cloud-fan]

LGTM

[SparkQA]

Test build #55803 has finished for PR 12386 at commit 7478aa5.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • class TaskUIData(
  • class ExecutorUIData(

[rxin]

Thanks - merging in master.