[SPARK-19055][SQL][PYSPARK] Fix SparkSession initialization when SparkContext is stopped

## What changes were proposed in this pull request?

In SparkSession initialization, we store created the instance of SparkSession into a class variable _instantiatedContext. Next time we can use SparkSession.builder.getOrCreate() to retrieve the existing SparkSession instance.

However, when the active SparkContext is stopped and we create another new SparkContext to use, the existing SparkSession is still associated with the stopped SparkContext. So the operations with this existing SparkSession will be failed.

We need to detect such case in SparkSession and renew the class variable _instantiatedContext if needed.

## How was this patch tested?

New test added in PySpark.

Please review http://spark.apache.org/contributing.html

 before opening a pull request.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #16454 from viirya/fix-pyspark-sparksession.

(cherry picked from commit c6c37b8a)
Signed-off-by: Wenchen Fan <wenchen@databricks.com>