Wrong magic number for 2019/01/01

Hi there
I just stumbled across this [line of code](https://github.com/ArduPilot/MAVProxy/blob/bade28014144a755ba33be0f3968c617185a37a4/MAVProxy/modules/mavproxy_OpenDroneID.py#L168C34-L168C34) and was wondering if it shouldn't be

```python
from datetime import datetime
from datetime import timezone

jan_1_2019_s = datetime(year=2019, month=1, day=1, tzinfo=timezone.utc).timestamp()
```

Because the referenced magic number 1546261200 is Mon Dec 31 2018 13:00:00 GMT+0000 in UTC. I also think the provided example is more readable. Maybe someone could consider to update this? If I find time I will provide a PR.

Open MAVProxy/modules/mavproxy_OpenDroneID.py at line 168 and inspect how the 2019-01-01 timestamp is defined and used. Replace the incorrect magic number with the UTC datetime-based value shown in the issue, then verify that the resulting timestamp is correct and the module remains valid Python.

Written by the indexing model from the issue text.