Summary:
We should remove all usages of React's legacy context API because it'll be removed from React at some point, it prevents some performance optimizations in updates and can cause conflicts between different context providers (like mixins).
This creates a new Context for `rootTag` (this granularity is intentional) so users that are consuming it via the legacy context API can start migrating away from it.
I didn't create a more generic context (like ReactRootContext, ReactApplicationContext) because having a more granular context makes it easier to track and remove it if we want to, and prevents re-rendering when users only care about certain values.
Reviewed By: rickhanlonii, cpojer
Differential Revision: D14941918
fbshipit-source-id: 7ceea62727d10a591367b7ed7c447309b286758d