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/Screens/RoomScreen/RoomScreenModels.swift

@@ -46,6 +46,7 @@ struct RoomScreenViewState: BindableState {
     
     var canJoinCall = false
     var hasOngoingCall: Bool
+    var shouldShowCallButton = true
     
     var bindings: RoomScreenViewStateBindings
 }