[native] Memorize ChatContextProvider

Summary:
D8847 changes ChatContextProvider position in root.react.js. We can also wrap this provider in React.memo.

Depends on D8847.

Test Plan: Run native app and check if everything works properly.

Reviewers: tomek, inka

Reviewed By: tomek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8874