Add ViewObservables.listViewScroll(AbsListView).

Would like to get @JakeWharton's thoughts as well. I know that a team member of yours (loganj?) had similar problems with this recently, where exceptions would end up being swallowed by ExecutorScheduler unless you manually ensured they were redirected to crash your app (and hence end up in the uncaught exception handler.) I believe he contributed a fix for it too. But it illustrates the same point.