[SPARK-9167][SQL] use UTC Calendar in `stringToDate`

fix 2 bugs introduced in https://github.com/apache/spark/pull/7353

1. we should use UTC Calendar when cast string to date . Before #7353 , we use `DateTimeUtils.fromJavaDate(Date.valueOf(s.toString))` to cast string to date, and `fromJavaDate` will call `millisToDays` to avoid the time zone issue. Now we use `DateTimeUtils.stringToDate(s)`, we should create a Calendar with UTC in the begging.
2. we should not change the default time zone in test cases. The `threadLocalLocalTimeZone` and `threadLocalTimestampFormat` in `DateTimeUtils` will only be evaluated once for each thread, so we can't set the default time zone back anymore.

Author: Wenchen Fan <cloud0fan@outlook.com>

Closes #7488 from cloud-fan/datetime and squashes the following commits:

9cd6005 [Wenchen Fan] address comments
21ef293 [Wenchen Fan] fix 2 bugs in datetime