[SPARK-18613][ML] make spark.mllib LDA dependencies in spark.ml LDA private

## What changes were proposed in this pull request?
spark.ml.*LDAModel classes were exposing spark.mllib LDA models via protected methods. Made them package (clustering) private.

## How was this patch tested?
```
build/sbt doc  # "millib.clustering" no longer appears in the docs for *LDA* classes
build/sbt compile  # compiles
build/sbt
> mllib/testOnly   # tests pass
```

Author: sueann <sueann@databricks.com>

Closes #16860 from sueann/SPARK-18613.