[native] Move ThreadAvatar "inside" EditThreadAvatar

Summary:
Same as D7508, but for EditThreadAvatar instead of EditUserAvatar.

Depends on D7508

Test Plan:
Things continue to look as expected:

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7509