Should only be returning an interaction controller if we intend to be interactive

[jverkoey]

We currently always treat every transition as an interactive transition. This is somewhat against the intended use of UIKit's transitioning APIs.

What we should be doing is acting as an interactive transition if there is some indication that the transition is, in fact, interactive. One signal we can use to start is whether any gesture recognizers associated with the transition are active.

Until we start working on interactive transitions support (Milestone 2) we may want to consider simplifying the transition controller logic to only implement the animated API variants.

[jverkoey]

🎊 My diff is ready for review at http://codereview.cc/D1736