Consistent dismissal behavior for Alert.alert()

[JonnyBurger]

I feel like it is okay that the alerts look differently on iOS and Android, I would not change it visually.

When tapping outside of the alert, the alert gets dismissed on Android but not on iOS:
https://facebook.github.io/react-native/docs/alert#android

I would make it so that the alert is not dismissable by default, because I suspect that many React Native developers develop on iOS and do not consider the case that on Android the prompt is dismissable.