Fully support Android 11 Conversations

It seems like there are a few things that need to be done to fully support Android 11's new Conversations interface:

- [ ] Enable conversation-specific features (priority conversations, per-contact ringtones, etc), as described in the [relevant documentation](https://developer.android.com/preview/features/conversations), by associating the notifications with a long-lived shortcut. (I think Element is currently in the "MessagingStyle is used but no shortcut is provided" stage)
- [ ] Crop room icons into a circle. As of now, room icons are square in Conversation notifications while user icons are circular, which looks a bit weird especially since room icons are circular in the app
- [ ] Ungroup separate chat notifications. It seems from this sample screenshot from the docs that the Android people ideally want ungrouped notifications:
Not to mention it looks a lot better when they're not grouped. Here are some examples of what Element/RiotX notifications actually look like on Android 11:
(the last two are from an older version, but the notification style hasn't changed at all since then)
It might make sense to carry this down to older Android versions too. I don't see why notifications from separate rooms need to be grouped together; many other messaging platforms don't do this.
- [ ] As of now, when I have Element/RiotX notifications from multiple rooms and I click on one, the grouped notification stays the same and doesn't dismiss the notification from the room I clicked on until I view the unread messages from *all* rooms. (this might be fixed by items 1 and/or 3 in this list - I have no idea but it seems likely)
- [ ] Support bubbles (#1124)