[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