Use native video call picture in picture! (#3190)

* Use native video call picture in picture!

* Handle isSupported, isPossible and web view errors.

* Use new canEnterPip method.

* Hide a room's Call button when already joined to the call.

* Tweak feature flag description.

* PR comments.

ElementX/Sources/Services/ElementCall/ElementCallConfiguration.swift

@@ -77,7 +77,7 @@ struct ElementCallConfiguration {
     }
     
     /// A string representing the call being configured.
-    var callID: String {
+    var callRoomID: String {
         switch kind {
         case .genericCallLink(let url):
             url.absoluteString

ElementX/Sources/Services/ElementCall/ElementCallService.swift

@@ -57,9 +57,14 @@ class ElementCallService: NSObject, ElementCallServiceProtocol, PKPushRegistryDe
     
     private var endUnansweredCallTask: Task<Void, Never>?
     
-    private var ongoingCallID: CallID?
+    private var ongoingCallID: CallID? {
+        didSet { ongoingCallRoomIDSubject.send(ongoingCallID?.roomID) }
+    }
     
-    var ongoingCallRoomID: String? { ongoingCallID?.roomID }
+    let ongoingCallRoomIDSubject = CurrentValueSubject<String?, Never>(nil)
+    var ongoingCallRoomIDPublisher: CurrentValuePublisher<String?, Never> {
+        ongoingCallRoomIDSubject.asCurrentValuePublisher()
+    }
     
     private let actionsSubject: PassthroughSubject<ElementCallServiceAction, Never> = .init()
     var actions: AnyPublisher<ElementCallServiceAction, Never> {

ElementX/Sources/Services/ElementCall/ElementCallServiceProtocol.swift

@@ -26,7 +26,7 @@ enum ElementCallServiceAction {
 protocol ElementCallServiceProtocol {
     var actions: AnyPublisher<ElementCallServiceAction, Never> { get }
     
-    var ongoingCallRoomID: String? { get }
+    var ongoingCallRoomIDPublisher: CurrentValuePublisher<String?, Never> { get }
     
     func setClientProxy(_ clientProxy: ClientProxyProtocol)