Implement call holding (call swapping still broken).

- Alice calls Bob on Signal and they start talking
- Charlie calls Alice on Not-Signal.
- Alice chooses to "Hold & Accept" putting Bob on Hold while the call with
  Charlie connects.
- If Alice ends the call with Charlie, we're back in Signal-iOS and
  talking to Bob, no problem.
- However, if, before ending the call with Charlie, Alice tries to swap
  *back* to bob, bob won't hear any audio in the callkit screen. Alice
  has to switch back to the Signal screen before the audio is transmitted.

// FREEBIE

Signal/src/call/SignalCall.swift

@@ -133,6 +133,8 @@ protocol CallObserver: class {
         }
     }
 
+    var isOnHold = false
+
     var connectedDate: NSDate?
 
     var error: CallError?

Signal/src/call/Speakerbox/CallKitCallUIAdaptee.swift

@@ -285,20 +285,19 @@ final class CallKitCallUIAdaptee: NSObject, CallUIAdaptee, CXProviderDelegate {
             action.fail()
             return
         }
-        Logger.warn("TODO, unimplemented set held call: \(call)")
 
-        // TODO FIXME
-        //        // Update the SpeakerboxCall's underlying hold state.
-        //        call.isOnHold = action.isOnHold
-        //
-        //        // Stop or start audio in response to holding or unholding the call.
-        //        if call.isOnHold {
-        //            // stopAudio() <-- SpeakerBox
-        //            PeerConnectionClient.stopAudioSession()
-        //        } else {
-        //            // startAudio() <-- SpeakerBox
-        //            PeerConnectionClient.startAudioSession()
-        //        }
+        // Update the SignalCall's underlying hold state.
+        call.isOnHold = action.isOnHold
+
+        // Stop or start audio in response to holding or unholding the call.
+        if call.isOnHold {
+            // stopAudio() <-- SpeakerBox
+            PeerConnectionClient.stopAudioSession()
+        } else {
+            // startAudio() <-- SpeakerBox
+            // This is redundant with what happens in `provider(_:didActivate:)`
+            //PeerConnectionClient.startAudioSession()
+        }
 
         // Signal to the system that the action has been successfully performed.
         action.fulfill()