Tabs flicker on load

the tab bar items flicker on load when using a tabs screen, see movie on the url for details

I'd guess it's due to using `animated: true` https://github.com/airbnb/native-navigation/blob/master/lib/ios/native-navigation/ReactTabBarController.swift#L167 here,

https://www.dropbox.com/s/oxdi8t1r2fmqmie/native-navigation%20tabs%20flicker%20bug.m4v?dl=0

Start with lib/ios/native-navigation/ReactTabBarController.swift at the animated: true call around line 167, then reproduce the tabs-screen load described in the attached video. Compare the initial tab bar behavior with animation enabled and identify a change that prevents the items from flickering on load; done means the tab bar renders without the reported flicker.

Written by the indexing model from the issue text.