[TIMOB-17687] (3_4_X) iOS8: Refactor AlertDialogProxy for iOS8

Cherry Pick PR #6188 into 3_4_X

Fixes the following
[TIMOB-17687](https://jira.appcelerator.org/browse/TIMOB-17687)
[TIMOB-17719](https://jira.appcelerator.org/browse/TIMOB-17719)