Fix read message hiding with threads

The behavior between keeping freshly unread standalone messages (or
rather top-level thread roots, actually) and child messages in threads
was different. When deciding whether the whole thread should be kept, we
should look at the state including the "recent changes", not just the
current state, otherwise an entire thread disappears too early.

BUG: 394664
Change-Id: I308433854079bf0e39d5e7a3589401a626ffd6fa