Only intercept transitions that were initiated interactively

This fixes a keyboard flash bug caused by non-interactive transitions, such as modal presentations. Another way to fix modal transitions whould be to check `presentationStyle`, but from what I can tell we only need to handle interactive transitions anyway.

[`[UIViewControllerTransitionCoordinator initiallyInteractive]`](https://developer.apple.com/library/ios/documentation/uikit/reference/UIViewControllerTransitionCoordinatorContext_Protocol/Reference/Reference.html#//apple_ref/occ/intfm/UIViewControllerTransitionCoordinatorContext/initiallyInteractive) returns
true only if the transition is both animated and interactive, so it seems like the right thing to check.