decide which file to use at compile time - allows for web support

The dependencies were resolved at runtime, not at build time. This leads to ActionSheetIOS getting loaded on all platforms, avoiding this allows for the use of react-native-web.

Can we revive this and hopefully get this in?

I tried testing on web. It looks like ActionSheetCustom uses modal and react native web doesn't support modal. Need a different solution for now to support web.

@smithydll You can use this to enable modals: https://github.com/necolas/react-native-web/issues/1020#issuecomment-416227930

Calling @xiaoyann :)