Vehicle reports "lost manual control" when entering joystick configuration page

From @rjehangir:

> Bug in 0.4.2: When you open joystick configuration, the vehicle says, “Critical: lost manual control”. Maybe you just stop sending buttons but keep sending the message? I think it would be fine to still send joystick signals.

Reproduce the issue by opening the joystick configuration page while connected to a vehicle, then trace the page's handling of joystick signals and the manual-control warning. Done means entering configuration no longer triggers a false “lost manual control” report while joystick signals continue as expected.

Written by the indexing model from the issue text.