[SPARK-11199][SPARKR] Improve R context management story and add getOrCreate

* Changes api.r.SQLUtils to use ```SQLContext.getOrCreate``` instead of creating a new context.
* Adds a simple test

[SPARK-11199] #comment link with JIRA

Author: Hossein <hossein@databricks.com>

Closes #9185 from falaki/SPARK-11199.