[SPARK-13074][CORE] Add JavaSparkContext. getPersistentRDDs method

The "getPersistentRDDs()" is a useful API of SparkContext to get cached RDDs. However, the JavaSparkContext does not have this API.

Add a simple getPersistentRDDs() to get java.util.Map<Integer, JavaRDD> for Java users.

Author: Junyang <fly.shenjy@gmail.com>

Closes #10978 from flyjy/master.