[SPARK-14386][ML] Changed spark.ml ensemble trees methods to return concrete types

## What changes were proposed in this pull request?

In spark.ml, GBT and RandomForest expose the trait DecisionTreeModel in the trees method, but they should not since it is a private trait (and not ready to be made public). It will also be more useful to users if we return the concrete types.

This PR: return concrete types

The MIMA checks appear to be OK with this change.

## How was this patch tested?

Existing unit tests

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

Closes #12158 from jkbradley/hide-dtm.