SPARK-3716 [GraphX] Update Analytics.scala for partitionStrategy assignment 

Previously, when the val partitionStrategy was created it called a function in the Analytics object which was a copy of the PartitionStrategy.fromString() method. This function has been removed, and the assignment of partitionStrategy now uses the PartitionStrategy.fromString method instead. In this way, it better matches the declarations of edge/vertex StorageLevel variables.