device: Always use an internal cancellable for tasks

This will allow libfprint to cancel operations internally in the future.

If the internal cancellation method is used, then the private
current_cancellation_reason variable must be set to the GError. This
error will be returned when set.

tests/test-device-fake.h

@@ -32,6 +32,8 @@ struct _FpiDeviceFake
   gpointer        last_called_function;
   gboolean        return_action_error;
 
+  GCancellable   *ext_cancellable;
+
   GError         *ret_error;
   FpPrint        *ret_print;
   FpPrint        *ret_match;