Upon hiding a room timeline, unsubscribe to "own user read receipts" update loop

[kevinaboos]

After PR #206 was merged, there are a few remaining issues:

• when a RoomScreen is being closed/hidden, we should unsubscribe from the updates to our own user's read receipts, just like we do for typing notices and other things that are room-specific.
• Remove this strange design that requires a mutex to track which rooms we are currently subscribed to for the own user's read receipts: https://github.com/project-robius/robrix/pull/206/files#diff-00f66b4f3b77e42fe6218a0af9946800a84a6ae245183f6a25c8b07b11a2fde7R413