[SPARK-20955][CORE] Intern "executorId" to reduce the memory usage

## What changes were proposed in this pull request?

In [this line](https://github.com/apache/spark/blob/f7cf2096fdecb8edab61c8973c07c6fc877ee32d/core/src/main/scala/org/apache/spark/scheduler/cluster/CoarseGrainedSchedulerBackend.scala#L128), it uses the `executorId` string received from executors and finally it will go into `TaskUIData`. As deserializing the `executorId` string will always create a new instance, we have a lot of duplicated string instances.

This PR does a String interning for TaskUIData to reduce the memory usage.

## How was this patch tested?

Manually test using `bin/spark-shell --master local-cluster[6,1,1024]`. Test codes:
```
for (_ <- 1 to 10) { sc.makeRDD(1 to 1000, 1000).count() }
Thread.sleep(2000)
val l = sc.getClass.getMethod("jobProgressListener").invoke(sc).asInstanceOf[org.apache.spark.ui.jobs.JobProgressListener]
org.apache.spark.util.SizeEstimator.estimate(l.stageIdToData)
```
This PR reduces the size of `stageIdToData` from 3487280 to 3009744 (86.3%) in the above case.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #18177 from zsxwing/SPARK-20955.