Three small bugs

Three small bugs:

1. If the presenter was already sharing his desktop, and then a mobile client joined, the mobile client would not pick up the desk share stream (I introduced this bug when rewriting the services).
2. "Exit" button would show on iOS devices, even though there is no way to exit the application. This has been removed on iOS devices.
3. If a user left voice while still talking, the talking icon would remain.