Stabilize AsyncRDDActions and expose its methods in Java API

The AsyncRDDActions methods are currently the easiest way to determine Spark jobs' ids for use in progress-monitoring code (see SPARK-2636). AsyncRDDActions is currently marked as @Experimental; for 1.2, I think that we should stabilize this API and expose it in Java, too.

One concern is whether there's a better async API design that we should prefer over this one as our stable API; I had some ideas for a more general API in SPARK-3626 (discussed in much greater detail on GitHub: https://github.com/apache/spark/pull/2482) but decided against the more general API due to its confusing cancellation semantics. Given this, I'd be comfortable stabilizing our current API.