[macOS] Emit QBluetoothLocalDevice::hostModeStateChanged if possible

The macOS backend uses IOBluetoothHostController in its implementation. This class does not provide any ways to set the host mode, so QBluetoothLocalDevice::setHostMode() is effectively a no-op.

However, the user can toggle bluetooth on and off from the system settings. We need to investigate if we can detect such external adapter state changes and emit the QBluetoothLocalDevice::hostModeStateChanged() signal in response.

If there's no platform-specific API to detect the adapter state changes, we should at least explicitly mention in the documentation that the signal is not emitted on macOS.