SPARK-2745 [STREAMING] Add Java friendly methods to Duration class

tdas is this what you had in mind for this JIRA? I saw this one and thought it would be easy to take care of, and helpful as I use streaming from Java.

I could do the same for `Time`? Happy to do so.

Author: Sean Owen <sowen@cloudera.com>

Closes #2403 from srowen/SPARK-2745 and squashes the following commits:

5a9e706 [Sean Owen] Change "Duration" to "Durations" to avoid changing Duration case class API
bda301c [Sean Owen] Just delegate to Scala binary operator syntax to avoid scalastyle warning
7dde949 [Sean Owen] Disable scalastyle for false positives. Add Java static factory methods seconds(), minutes() to Duration. Add Java-friendly methods to Time too, and unit tests. Remove unnecessary math.floor from Time.floor()
4dee32e [Sean Owen] Add named methods to Duration in parallel to symbolic methods for Java-friendliness. Also add unit tests for Duration, in Scala and Java.