land detector: move arming state into base class & set param when disarming

Before that, different modules (ekf2, commander & land detector) changed
params upon different events:
- ekf2 & commander set params after disarm
- land detector set params on land detected
If the 2 events were several 100ms appart, it led to 2 param saves, and
the latter param set could have been blocked by an ongoing save. And if
the land detector was blocked, it could lead to mag timeouts.

This patch makes all modules use the same event, thus only a single param
save will happen.

If we want to have guarantees to never block, we should introduce a
param_try_set() API method.