Fix #3133 - Cancel ElementCall rining once answered elsewhere

- expose `activeRoomCallParticipants` and use them to detect if the user has joined the call from a different device

ElementX/Sources/Services/Room/RoomProxyProtocol.swift

@@ -41,7 +41,10 @@ protocol RoomProxyProtocol {
     var pinnedEventIDs: Set<String> { get async }
     var membership: Membership { get }
     var inviter: RoomMemberProxyProtocol? { get async }
+    
     var hasOngoingCall: Bool { get }
+    var activeRoomCallParticipants: [String] { get }
+    
     var canonicalAlias: String? { get }
     var ownUserID: String { get }