[lib] Replace threadInChatList with useThreadInChatList in useWatchThread

Summary:
Similar to rest of stack, but for useWatchThread.

Depends on D11980

Test Plan:
flow + close reading

Set log statements in useWatchThread and ensured that values before and after were identical.

Reviewers: ashoat, ginsu, tomek, varun, will

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D11981