[native] Naively split EditAvatar into EditUserAvatar and EditThreadAvatar

Summary:
We just copy the existing contents of EditAvatar and paste into EditUserAvatar and EditThreadAvatar and update imports.

In subsequent diffs we will:

1. Move UserAvatar and ThreadAvatar "into" EditUserAvatar and EditThreadAvatar respectively instead of passing in React.Node prop.
2. Pull out common functionality to hooks
3. Introduce user/avatar-specific functionality
4. Once everything is working, consider introducing some sort of BaseEditAvatar with some of the common styling/layout stuff (eg pencil icon). Defer this until the end once we have everything working.

Depends on D7491

Test Plan: App appears to work as before. Flow/ESLint to check imports.

Reviewers: ashoat, ginsu

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7492