ComplexCharacteristic does not pass fixed_length instance variable to _bleio.Characteristic.add_to_service

This does not pass a keyword arg of `fixed_length` and therefore picks up a `False` default value:

https://github.com/adafruit/Adafruit_CircuitPython_BLE/blob/e38b153dbcff10a72b7ab422882558e3a406e458/adafruit_ble/characteristics/__init__.py#L206-L214

The class does have an instance variable of `fixed_length` - it looks like this was intended to be passed?

Open adafruit_ble/characteristics/__init__.py at lines 206-214 and inspect ComplexCharacteristic's call to _bleio.Characteristic.add_to_service. Confirm how the instance's fixed_length value is available there, then ensure the call passes it as a keyword argument. Done means the instance value is used instead of the False default.

Written by the indexing model from the issue text.