private void free() {
		closedCallback = null;
		valueCallback = null;
		dataMerger = null;
		progressCallback = null;
		filter = null;
		packetFilter = null;
		buffer = null;
		count = 0;
	}
Sets a callback that will be executed when the device services were invalidated (i.e. on
disconnection) or the callback has been unregistered and it can release resources.

@param callback the callback.
@return The request.
ble/src/main/java/no/nordicsemi/android/ble/ValueChangedCallback.java