[SPARK-12115][SPARKR] Change numPartitions() to getNumPartitions() to be...

[SPARK-12115][SPARKR] Change numPartitions() to getNumPartitions() to be consistent with Scala/Python

Change ```numPartitions()``` to ```getNumPartitions()``` to be consistent with Scala/Python.
<del>Note: If we can not catch up with 1.6 release, it will be breaking change for 1.7 that we also need to explain in release note.<del>

cc sun-rui felixcheung shivaram

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #10123 from yanboliang/spark-12115.