Improve and fix iOS audio route management.

This version introcuces for iOS a new LinphoneAudioDevice instance with a specific "follows system routing" capability (see linphone_audio_device_get_follows_system_routing_policy()). This LinphoneAudioDevice becomes the default one when upgrading, and simply follows the system routing policy when handling calls. Indeed, it is not possible to know prior to a call whether a bluetooth device is available, so the best approach is to let the system device. Application can of course later override the audio route by setting other LinphoneAudioDevice representing speaker or bluetooth outputs.