boolean hasObservers() {
            return valueRelay.hasObservers() || errorRelay.hasObservers();
        }
{@link #setNativeCallback(BluetoothGattCallback)}
Since Android 8.0 (API 26) BluetoothGattCallback has some hidden method(s). Setting this {@link HiddenBluetoothGattCallback} will
relay calls to those hidden methods.

On API lower than 26 this method does nothing

@param callbackHidden the object to be called
rxandroidble/src/main/java/com/polidea/rxandroidble2/internal/connection/RxBleGattCallback.java