Ongoing call tiles - Implement the view-models and store

[MidhunSureshR]

Branched from #34197

This PR implements the view-models necessary for driving the views. There's no foolproof way to associate a given call to a rtc notification event (which is what we render as tiles) so the basic idea is to treat the last notification tile in the timeline as an ongoing call tile if there's a call in the room.

• There's a BaseOngoingCallTileViewModel that provides the shared snapshot content. DmOngoingCallTileViewModel and RoomOngoingCallTileViewModel both extend this base view model to provide the part of the snapshot that is not common.
• Implements FacePileViewModel, MemberAvatarViewModel and DurationViewModel for driving the views from the last PR.
• There's a new store, LatestRtcNotificationEventStore, that tracks the latest notification event in a given room. This is necessary because the call store tells you about a new call as soon as a call membership event comes through but the timeline only renders rtc notification events. The problem arises when there's a delay between receiving the call membership event and the notification event. As a result, CallStore tells us that a call is ongoing in the room but the corresponding rtc notification event hasn't arrived, so we incorrectly show the last call tile (which in reality relates to a call that is already over) as ongoing.