AP_NavEKF: Track baro alt when pre-armed

This will help prevent spurious alt disparity warning messages for copter when EKF is selected.

The current EKF behaviour is to track a zero height before being armed, and to track the last baro reading when disarmed. This is different behaviour to INAV and causes the consistency check between Baro and Inertial height to fail if baro height drifts from start-up before it can be armed or after disarming.