[lib] introduce getUserAvatarForThread function

Summary:
The getUserAvatarForThread function returns the user avatar for threads that are either PERSONAL or PRIVATE. We inject the user avatars in threadInfoFromRawThreadInfo so that it is already set, and when we later call useGetAvatarForThread (introduced in D7132) we will just return that

Depends on D7177

Test Plan:
Please see the screenshots below to see that the correct user avatar is being rendered on to a PERSONAL and PRIVATE thread

For reference my friend's avatar:

For reference my avatar:

Personal Threads:

Private Thread:

Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7178