[SPARK-15259] Sort time metric should not include spill and record insertion time

## What changes were proposed in this pull request?

After SPARK-14669 it seems the sort time metric includes both spill and record insertion time. This makes it not very useful since the metric becomes close to the total execution time of the node.

We should track just the time spent for in-memory sort, as before.

## How was this patch tested?

Verified metric in the UI, also unit test on UnsafeExternalRowSorter.

cc davies

Author: Eric Liang <ekl@databricks.com>
Author: Eric Liang <ekhliang@gmail.com>

Closes #13035 from ericl/fix-metrics.