[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.

R/pkg/inst/tests/testthat/test_sparkSQL.R

@@ -62,6 +62,10 @@ mockLinesComplexType <-
 complexTypeJsonPath <- tempfile(pattern="sparkr-test", fileext=".tmp")
 writeLines(mockLinesComplexType, complexTypeJsonPath)
 
+test_that("calling sparkRSQL.init returns existing SQL context", {
+  expect_equal(sparkRSQL.init(sc), sqlContext)
+})
+
 test_that("infer types and check types", {
   expect_equal(infer_type(1L), "integer")
   expect_equal(infer_type(1.0), "double")

sql/core/src/main/scala/org/apache/spark/sql/api/r/SQLUtils.scala

@@ -32,7 +32,7 @@ private[r] object SQLUtils {
   SerDe.registerSqlSerDe((readSqlObject, writeSqlObject))
 
   def createSQLContext(jsc: JavaSparkContext): SQLContext = {
-    new SQLContext(jsc)
+    SQLContext.getOrCreate(jsc.sc)
   }
 
   def getJavaSparkContext(sqlCtx: SQLContext): JavaSparkContext = {