Fix display addCardViewControllerFooterView

[hico17]

Summary

For what we have seen, If you set the addCardViewControllerFooterView property inside the STPPaymentOptionsViewController after the commonInit call, the same property in the inner view controllers will never be set.

Solution

Take the addCardViewControllerFooterView value, used in the commonInit, from the paymentContext.

Motivation

In our application we have seen that the footer view was never displayed.

Testing

To test it, we just set addCardViewControllerFooterView in the payment context, it will never be displayed before the change.

[davidme-stripe]

Thank you for the PR, and sorry for the delay in screening it! We'll try to include this in a future update, though I need to think through some of the implications of how the FooterView is used in other contexts.