[SPARK-20088] Do not create new SparkContext in SparkR createSparkContext

## What changes were proposed in this pull request?
Instead of creating new `JavaSparkContext` we use `SparkContext.getOrCreate`.

## How was this patch tested?
Existing tests

Author: Hossein <hossein@databricks.com>

Closes #17423 from falaki/SPARK-20088.