Currently, when we receive a room key bundle to-device event, we don't look up
the sender device at all, meaning that the message is then marked as "from
missing device", which means that if you turn on "exclude insecure devices",
the message is dropped.
This patch changes the logic so that room key bundle to-device events are
treated the same way as most other to-device events (except room keys, which
continue to be special).
Fixes: https://github.com/matrix-org/matrix-rust-sdk/issues/5613, although the
integration test now fails because instead we hit https://github.com/matrix-org/matrix-rust-sdk/issues/5768.
Richard van der Hoff
43e94bcfb4