[SPARK-11842][ML] Small cleanups to existing Readers and Writers

Updates:
* Add repartition(1) to save() methods' saving of data for LogisticRegressionModel, LinearRegressionModel.
* Strengthen privacy to class and companion object for Writers and Readers
* Change LogisticRegressionSuite read/write test to fit intercept
* Add Since versions for read/write methods in Pipeline, LogisticRegression
* Switch from hand-written class names in Readers to using getClass

CC: mengxr

CC: yanboliang Would you mind taking a look at this PR?  mengxr might not be able to soon.  Thank you!

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #9829 from jkbradley/ml-io-cleanups.