[Element Call] Implement 'ongoing call' UI changes in room list and chat screen

[jmartinesp]

With matrix-org/matrix-rust-sdk#2411 we can now subscribe to changes in the RoomInfo for a room. This will allow us to keep the screens that use a Room updated with its latest state.

For Element Call, although the designs are still being worked in, this should imply:

• Tracking some isOngoingCall parameter from RoomInfo (not available when this was written).
• Adding a create/join call icon button to the chat screen's app bar, and set its state based on this parameter.
• Same as above, add some indicator to the room list item that an ongoing call is happening in that room.

[jmartinesp]

The first 2 items were done in #1640.