Fixed an issue where the unread count would disappear too fast

Especially with images this was leading to really bad transitions
since the image would pop over immediately.
We now update the counter once the expansion has finished

Fixes: 153392205
Test: add conversations with messages with unread count and image, click on it.
Change-Id: I2ad2ec6a989a21154e3edbf09aabc6d17ea75bd7