[native] Replace scrollPos instance variable w/ ref

Summary:
As part of converting ChatThreadList to a functional component we'll need to replace the scrollPos instance variable with a ref.

Depends on D9135

Test Plan:
Things seem to continue to work as expected. Haven't used useRef a whole lot, so would be good to get feedback from reviewers w/ more familiarity.

Logging values seems to result in correct values:

Reviewers: ashoat, ginsu, rohan, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D9136