[CommCoreModule] Handle thread.avatar in parseDBThreads

Summary: So we can "pull out" avatar stored in SQLite.

Test Plan: Log jsiThreads in parseDBThreads and check return value of CommCoreModule.getAllThreads(...) on the JSI side.

Reviewers: ginsu, ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7290