[native] Use a JSON blob for heightMeasurerKey

Summary: Context on Linear.

Test Plan:

1. Make sure we don't see height measurement errors when running the app on iOS simulator
2. Make sure that messages actually render when running the app on iOS simulator

Reviewers: atul, ginsu

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7093