[SPARK-19377][WEBUI][CORE] Killed tasks should have the status as KILLED

## What changes were proposed in this pull request?

Copying of the killed status was missing while getting the newTaskInfo object by dropping the unnecessary details to reduce the memory usage. This patch adds the copying of the killed status to newTaskInfo object, this will correct the display of the status from wrong status to KILLED status in Web UI.

## How was this patch tested?

Current behaviour of displaying tasks in stage UI page,

| Index | ID | Attempt | Status | Locality Level | Executor ID / Host | Launch Time | Duration | GC Time | Input Size / Records | Write Time | Shuffle Write Size / Records | Errors |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
|143	|10	|0	|SUCCESS	|NODE_LOCAL	|6 / x.xx.x.x stdout stderr|2017/01/25 07:49:27	|0 ms |		|0.0 B / 0		| |0.0 B / 0	|TaskKilled (killed intentionally)|
|156	|11	|0	|SUCCESS	|NODE_LOCAL	|5 / x.xx.x.x stdout stderr|2017/01/25 07:49:27	|0 ms |		|0.0 B / 0		| |0.0 B / 0	|TaskKilled (killed intentionally)|

Web UI display after applying the patch,

| Index | ID | Attempt | Status | Locality Level | Executor ID / Host | Launch Time | Duration | GC Time | Input Size / Records | Write Time | Shuffle Write Size / Records | Errors |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
|143	|10	|0	|KILLED	|NODE_LOCAL	|6 / x.xx.x.x stdout stderr|2017/01/25 07:49:27	|0 ms |		|0.0 B / 0		|  | 0.0 B / 0	| TaskKilled (killed intentionally)|
|156	|11	|0	|KILLED	|NODE_LOCAL	|5 / x.xx.x.x stdout stderr|2017/01/25 07:49:27	|0 ms |		|0.0 B / 0		|  |0.0 B / 0	| TaskKilled (killed intentionally)|

Author: Devaraj K <devaraj@apache.org>

Closes #16725 from devaraj-kavali/SPARK-19377.

(cherry picked from commit df4a27cc)
Signed-off-by: Shixiong Zhu <shixiong@databricks.com>