[lib] Update getDefaultAvatar and threadMembersWithoutAddedAshoat

Summary:
issue: https://linear.app/comm/issue/ENG-6907/update-getdefaultavatar-and-threadmemberswithoutaddedashoat
getDefaultAvatar should strip any keyserver prefix
threadMembersWithoutAddedAshoat should remove any admin

Test Plan:
tested that threadMembersWithoutAddedAshoat correctly removes admin
tested that getDefaultAvatar correctly splits the string

Reviewers: michal, tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11146