[SPARK-19282][ML][SPARKR] RandomForest Wrapper and GBT Wrapper return param "maxDepth" to R models

## What changes were proposed in this pull request?

RandomForest R Wrapper and GBT R Wrapper return param `maxDepth` to R models.

Below 4 R wrappers are changed:
* `RandomForestClassificationWrapper`
* `RandomForestRegressionWrapper`
* `GBTClassificationWrapper`
* `GBTRegressionWrapper`

## How was this patch tested?

Test manually on my local machine.

Author: Xin Ren <iamshrek@126.com>

Closes #17207 from keypointt/SPARK-19282.