[keyserver] Update fetchKnownUserInfos to include avatar field in UserInfos

Summary: We want to make sure that we're populating the avatar field if set.

Test Plan:
Logged results from query and observed that "other" user avatar was populated in Redux:

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D7203