Summary:
Fixes https://github.com/facebook/react-native/issues/26830 by removing version gating around `RCTUserInterfaceStyleDidChangeNotification` sent by `RCTRootView` and observing that notif for `Dimensions` changes.
Also centralizes `RCTUserInterfaceStyleDidChangeNotification` constant definition in new `RCTConstants` file.
Changelog:
[iOS] [Fixed] - `Dimensions` module now updates on initial split screen
Reviewed By: sammy-SC
Differential Revision: D18931098
fbshipit-source-id: e9784be3f544f3b10360fbc2d6ad0324273b1a8f