Fix public read receipts being sent by mistake (#6838)
When returning to the chat screen from the room details one or a member's profile, `TimelineEvent.OnScrollFinished` will be called immediately, and this would read the default value for `isSendPublicReadReceiptsEnabled`, which is `true`.
If you had public read receipts disabled, this is a mistake, and would send a public read receipt. Instead, what we want to do is wait until the updated value is emitted and use it to decide whether we want to send a public or private read receipt.