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/RoomProxy.swift

@@ -123,6 +123,10 @@ class RoomProxy: RoomProxyProtocol {
         room.hasActiveRoomCall()
     }
     
+    var activeRoomCallParticipants: [String] {
+        room.activeRoomCallParticipants()
+    }
+    
     var canonicalAlias: String? {
         room.canonicalAlias()
     }

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 }