[MINOR] Revert removing explicit typing (changed in some examples and StatFunctions)

## What changes were proposed in this pull request?

This PR reverts some changes in https://github.com/apache/spark/pull/12413. (please see the discussion in that PR).

from
```scala
    words.foreachRDD { (rdd, time) =>
    ...
```

to
```scala
    words.foreachRDD { (rdd: RDD[String], time: Time) =>
    ...
```

Also, this was discussed in dev-mailing list, [here](http://apache-spark-developers-list.1001551.n3.nabble.com/Question-about-Scala-style-explicit-typing-within-transformation-functions-and-anonymous-val-td17173.html)

## How was this patch tested?

This was tested with `sbt scalastyle`.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #12452 from HyukjinKwon/revert-explicit-typing.