tracker: release manual control when setting mode

[robertlong13]

When I was testing https://github.com/ArduPilot/ardupilot/pull/30202, it took me a while to figure out how to release manual control. Once you do tracker position, you cannot get the tracker to do anything else until you call tracker position 32767 32767.

I think it's probably appropriate to release manual control any time the user changes mode, so that's what I've done here.