[SPARK-32068][WEBUI] Correct task lauchtime show issue due to timezon…

…e in stage tab

### What changes were proposed in this pull request?
`formatDate` in utils.js `org/apache/spark/ui/static/utils.js` is partly refactored.

### Why are the changes needed?
In branch-2.4，task launch time is returned as html string from driver,
while in branch-3.x，this is returned in JSON Object as`Date`type  from `org.apache.spark.status.api.v1.TaskData`
Due to：
LaunchTime from jersey server in spark driver is correct, which will be converted to date string like `2020-06-28T02:57:42.605GMT` in json object, then the formatDate in utils.js treat it as date.split(".")[0].replace("T", " ").
So `2020-06-28T02:57:42.605GMT` will be converted to `2020-06-28 02:57:42`, but correct is `2020-06-28 10:57:42` in GMT+8 timezone.
![选区_071](https://user-images.githubusercontent.com/7149304/85937186-b6d36780-b933-11ea-8382-80a3891f1c2a.png)
![选区_070](https://user-images.githubusercontent.com/7149304/85937190-bcc94880-b933-11ea-8860-2083c97ea269.png)

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
Manual test.

Closes #28918 from TJX2014/master-SPARK-32068-ui-task-lauch-time-tz.

Authored-by: TJX2014 <xiaoxingstack@gmail.com>
Signed-off-by: Thomas Graves <tgraves@apache.org>
(cherry picked from commit 165c948)
Signed-off-by: Thomas Graves <tgraves@apache.org>

core/src/main/resources/org/apache/spark/ui/static/utils.js

@@ -56,13 +56,17 @@ function formatTimeMillis(timeMillis) {
     return "-";
   } else {
     var dt = new Date(timeMillis);
+    return formatDateString(dt);
+  }
+}
+
+function formatDateString(dt) {
     return dt.getFullYear() + "-" +
       padZeroes(dt.getMonth() + 1) + "-" +
       padZeroes(dt.getDate()) + " " +
       padZeroes(dt.getHours()) + ":" +
       padZeroes(dt.getMinutes()) + ":" +
       padZeroes(dt.getSeconds());
-  }
 }
 
 function getTimeZone() {
@@ -161,7 +165,10 @@ function setDataTableDefaults() {
 
 function formatDate(date) {
   if (date <= 0) return "-";
-  else return date.split(".")[0].replace("T", " ");
+  else {
+     var dt = new Date(date.replace("GMT", "Z"))
+     return formatDateString(dt);
+  }
 }
 
 function createRESTEndPointForExecutorsPage(appId) {