[native] Convert ComposeThreadButton to a hook

Summary:
This diff should be a no-op. In a later diff, I need to update the createPendingThread call here. I figured the component made more sense as a hook and it was a quick refactor.

Depends on D6756

Test Plan: Flow, and playing around with thread composer on native

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6757