[SPARK-10076] [ML] make MultilayerPerceptronClassifier layers and weights public

Fix the issue that ```layers``` and ```weights``` should be public variables of ```MultilayerPerceptronClassificationModel```. Users can not get ```layers``` and ```weights``` from a ```MultilayerPerceptronClassificationModel``` currently.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #8263 from yanboliang/mlp-public.

mllib/src/main/scala/org/apache/spark/ml/classification/MultilayerPerceptronClassifier.scala

@@ -172,8 +172,8 @@ class MultilayerPerceptronClassifier(override val uid: String)
 @Experimental
 class MultilayerPerceptronClassificationModel private[ml] (
     override val uid: String,
-    layers: Array[Int],
-    weights: Vector)
+    val layers: Array[Int],
+    val weights: Vector)
   extends PredictionModel[Vector, MultilayerPerceptronClassificationModel]
   with Serializable {