Summary:
Changelog: [Internal]
Since D22098586 (https://github.com/facebook/react-native/commit/476ab7481e23070fc4db3f584e45a95eb2f9f7e1) zIndex is only applied to views with position other than static.
Sticky header however needs to have its zIndex applied otherwise it goes below the content.
Reviewed By: mdvacca
Differential Revision: D22843134
fbshipit-source-id: d2e3a21441795b82c44b6d4245ccf89620fd9a8e