[TIMOB-13901] iOS: Add Ti.UI.SIZE support for ListItemProxy

Fixes [TIMOB-13901](https://jira.appcelerator.org/browse/TIMOB-13901)
Ti.UI.SIZE support for custom templates (not the built in ones)
Note this functionality is not guaranteed to work in templates which load remote images.

Also fixes the issue reported in [TIMOB-666]((https://jira.appcelerator.org/browse/TIMOB-13901)) for listView. Does not fix it for tableView so do not resolve the ticket