update call screen avatar when contacts change

in particular this is a problem when initiating a call from a cold app
state. The call screen renders before the contacts have completely
loaded.

// FREEBIE

Signal/src/ViewControllers/CallViewController.swift

@@ -146,7 +146,11 @@ class CallViewController: UIViewController, CallObserver, CallServiceObserver, R
         createViews()
 
         contactNameLabel.text = contactsManager.displayName(forPhoneIdentifier: thread.contactIdentifier())
-        contactAvatarView.image = OWSAvatarBuilder.buildImage(for: thread, contactsManager: contactsManager, diameter:400)
+        updateAvatarImage()
+        NotificationCenter.default.addObserver(forName: .OWSContactsManagerSignalAccountsDidChange, object: nil, queue: nil) { _ in
+            Logger.info("\(self.TAG) updating avatar image")
+            self.updateAvatarImage()
+        }
 
         assert(call != nil)
         // Subscribe for future call updates
@@ -314,6 +318,10 @@ class CallViewController: UIViewController, CallObserver, CallServiceObserver, R
         button.setImage(image, for:.selected)
     }
 
+    func updateAvatarImage() {
+        contactAvatarView.image = OWSAvatarBuilder.buildImage(for: thread, contactsManager: contactsManager, diameter:400)
+    }
+
     func createIncomingCallControls() {
 
         acceptIncomingButton = createButton(imageName:"call-active-wide",