Fix the oxygen percentage for air dives.
Fix the oxygen percentage for air dives.

When the dive mode setting is set to air, the oxygen percentage stored
in the header is different from the expected 21%. It might be the last
used nitrox percentage.

glance-/libdivecomputer,venkateshshukla/libdivecomputer,josh-wambua/libdivecomputer,henrik242/libdivecomputer,henrik242/libdivecomputer,venkateshshukla/libdivecomputer,Poltsi/libdivecomputer-vms,glance-/libdivecomputer,josh-wambua/libdivecomputer