Room preview displays reactions as messages

As per https://github.com/vector-im/riot-web/issues/14298, the room list in Element Web doesn't include reactions in the preview of a room anymore. It would be nice to have that change ported to the Android app as well (since I see that it's showing reactions in room previews) because it can lead to a confusing UX otherwise (see the issue on Element Web).

Start by reading the linked Element Web issue 14298 to understand the intended room-preview behavior, then locate the Android room-list code that assembles preview text. Done means reactions no longer appear as messages in Android room previews, with relevant coverage updated if the project has a suitable test.

Written by the indexing model from the issue text.