[TIMOB-15283] (3_1_X) iOS: SplitWindow does not respond to focus events

Cherry Pick PR #4740 into 3_1_X

Fixes [TIMOB-15283](https://jira.appcelerator.org/browse/TIMOB-15283)

Test case is in [comments](https://jira.appcelerator.org/browse/TIMOB-15283#comment-273034)