SPARK-4297 [BUILD] Build warning fixes omnibus

There are a number of warnings generated in a normal, successful build right now. They're mostly Java unchecked cast warnings, which can be suppressed. But there's a grab bag of other Scala language warnings and so on that can all be easily fixed. The forthcoming PR fixes about 90% of the build warnings I see now.
I think that this Py4J unzipping / copying is only needed when building for PySpark on YARN:

https://spark.apache.org/docs/latest/building-spark.html#building-for-pyspark-on-yarn

So, this isn't necessary for tests.