[native] Pull displayFailureAlert out of EditThreadAvatarProvider

Summary:
Similar to D8213, but for thread avatars (changes aren't 1:1 for both providers so separated changes even if there's a little redundancy... also the diffs should be less noisy so easier to review?)

Depends on D8217 (kinda)

Test Plan: Killed keyserver and made sure alert appeared as expected.

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8218