[SPARK-17499][SPARKR][ML][MLLIB] make the default params in sparkR spark.mlp consistent with MultilayerPerceptronClassifier

## What changes were proposed in this pull request?

update `MultilayerPerceptronClassifierWrapper.fit` paramter type:
`layers: Array[Int]`
`seed: String`

update several default params in sparkR `spark.mlp`:
`tol` --> 1e-6
`stepSize` --> 0.03
`seed` --> NULL ( when seed == NULL, the scala-side wrapper regard it as a `null` value and the seed will use the default one )
r-side `seed` only support 32bit integer.

remove `layers` default value, and move it in front of those parameters with default value.
add `layers` parameter validation check.

## How was this patch tested?

tests added.

Author: WeichenXu <WeichenXu123@outlook.com>

Closes #15051 from WeichenXu123/update_py_mlp_default.