Fix onDismiss in Modal

Summary:
# Disclaimer:
I might be missing something as the solution I implemented here seems like something that was considered by original author. If this solution isn't good, I have a plan B.

# Problem:
`onDismiss` prop isn't being called once the modal is dismissed, this diff fixes it.

Also I've noticed that `onDismiss` is meant to only work on iOS, why is that? By landing this diff, it'll be called on Android as well so we need to change the docs (https://facebook.github.io/react-native/docs/modal.html#ondismiss).

## Video that shows the problem
Following code is in playground.js P70222409 which just increments number everytime onDismiss is called

{F166303269}

Reviewed By: shergin

Differential Revision: D16109536

fbshipit-source-id: 3fba56f5671912387b217f03b613dffd89614c9d