[lib] Convert getMessagePreview to hook

Summary:
Since it contains usernames, we'll need to auto-update it when the ENS name comes in. So we need a hook.

Next diff will add ENS fetching functionality to useMessagePreview.

Depends on D6374

Test Plan:
Confirmed that all uses of getMessagePreview still worked:

1. Subchannel/sidebar modal on web
2. MessagePreview on both native and web

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6375