Add ViewObservables.listViewScroll(AbsListView).

I always thought these double dispatches are weird. We're already testing as a precondition that we're on the main thread, so unless we're rescheduling the observable purposefully to deliver notifications on a background thread after this operator is applied, we will also unsubscribe on the main thread.

I've seen this being done a lot in the last few PRs and always wondered if we're just being defensive or if this will not simply cause an extra cycle through the message loop in the normal case, thus unnecessarily delaying unsubscription?