[TIMOB-18075] (3_5_X) Restore Focus to top WindowProxy

Cherry Pick PR #6417 to 3_5_X

Since alertDialogs/OptionDialogs are now UIAlertController presented
modally, their dismissal should ensure top most window proxy has focus.

Test is in [JIRA](https://jira.appcelerator.org/browse/TIMOB-18075)