AP_Avoidance: ADSB targets generally provide theire height as a pressure altitude

[WickedShell]

Bug report

ADSB targets generally provide height as a pressure altitude, however AP_Avoidance uses it as an absolute height. (This is not actually an AP_Avoidance bug, so much as it is how AP_ADSB ingests the data and tags it as absolute, however AP_ADSB should probably not be transforming the data, as we are responsible for later transmitting it out again, and applying our own transformations to height would be incorrect. We need to actually be looking at the ADSB_PRESSURE_TYPE enum that's in a MAVLink ADSB_VEHICLE report. If we got the ADSB data from a native driver the driver will need to be responsible for doing the correction.

The problem with this is that pressure altitude can easily be more then 500 feet off, which has a rather significant impact when trying to calculate vertical clearance in AP_Avoidance.

This was found by inspection, but I can't find any spot doing the correction, so I believe I'm correct.

Version
4.4 (but master appears to be the same).

Platform
[ x ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Submarine