Megolm session retrieved from backup incorrectly marked as insecure

Nobody has claimed this yet.

I gave up trying to reproduce this, either on EX or EW, though I can't see any particular reason it wouldn't happen on either. I think it requires a large enough room that the affected user isn't one of the room heroes, or something.

1. Observe a message for which we lack the keys, and for whose sender we lack device details.
2. Receive a megolm key from key backup. SenderData is marked as `UnknownDevice { legacy_session: true, owner_check_failed: false }`
3. Receive a keys/query update for the sending user. We call `update_sender_data_for_sessions_for_device`, which calls `SenderDataFinder::have_device`, which replaces the SenderData for imported sessions with `SenderData::UnknownDevice { legacy_session: false, owner_check_failed: true }`.
4. Since we now have `legacy_session: false`, the message is hidden when "hide messages from insecure devices" is enabled.

Start with SenderDataFinder::have_device and its caller update_sender_data_for_sessions_for_device, then trace how key-backup imports set SenderData. Reproduce the sequence described in the issue and verify that a retrieved Megolm session keeps the correct security status after the keys/query update, including when insecure-device hiding is enabled.

Written by the indexing model from the issue text.