[SPARK-11912][ML] ml.feature.PCA minor refactor

Like [SPARK-11852](https://issues.apache.org/jira/browse/SPARK-11852), ```k``` is params and we should save it under ```metadata/``` rather than both under ```data/``` and ```metadata/```. Refactor the constructor of ```ml.feature.PCAModel```  to take only ```pc``` but construct ```mllib.feature.PCAModel``` inside ```transform```.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #9897 from yanboliang/spark-11912.