application: Don't add a provider if one is already present

If we initially invoked `gnome-calls -p sip` and later invoke
`gnome-calls sip:user@host` we don't expect to have the default MM provider
added.

src/calls-manager.h

@@ -66,5 +66,6 @@ GListModel            *calls_manager_get_suitable_origins     (CallsManager
 const gchar           *calls_manager_get_contact_name         (CallsCall        *call);
 gboolean               calls_manager_has_active_call          (CallsManager     *self);
 void                   calls_manager_hang_up_all_calls        (CallsManager     *self);
+gboolean               calls_manager_has_any_provider         (CallsManager *self);
 
 G_END_DECLS