altosui: Report baro ground alt for 'Pad Altitude' when GPS missing

altosui: Report baro ground alt for 'Pad Altitude' when GPS missing

The 'Pad Altitude' field in the pad tab of AltosUI was originally one
of three fields designed to report the GPS position of the pad. When
TeleMini was added, the code was updated to separate the lat/lon from
altitude display, but the pad_alt field in AltosState used for that
was still only computed from GPS data.

This fix gets rid of the confusing 'pad_alt' field in AltosState and
has the 'Pad Altitude' data select either GPS ground altitude or baro
ground altitude, depending on whether GPS data is available.

signed-off-by: Keith Packard <keithp@keithp.com>