Commander: improve message for RC override event

[MaEtUgR]

Describe problem solved by the proposed pull request
@cmic0 just discussed with me over lunch that "Autopilot off" is a really bad thing to hear when flying an aircraft since that means the controller you need for normal flight is gone and it's an emergency. Also for a multicopter you could interpret it as the autopilot hardware turning off which is totally undesired in flight.

Describe your preferred solution
I suggest (it was @cmic0 's idea) to just say "Autonomy off" because it's far less mistakable.

[julianoes]

Better!

[cmic0]

Thanks for taking care @MaEtUgR!
Yes, having the autopilot off means that you don't even have primary flight modes running on the autopilot (for example rate stabilization) and you are purely flying in manual: this could be very risky if the aircraft is not intrinsically stable. Totally agree that on a multicopter this is highly undesired :)!
"Autonomy off" seems to be more clear.