Bareground area_pft not being carried over during restarts

[glemieux]

In testing ESCOMP/CTSM#1849, I stumbled on FATES_TVEG not being b4b during restarts. This is due to the zero index of area_pft not being set during the restart, which can be seen by writing out the site_area_veg variable during history write. Even though all the other indices of the area_pft value are carried over during the restart, the only place that area_pft is calculated (including the zero index) is within set_site_properties, which is only called during init_coldstart.

This also explains why FATES_TVEG is b4b for the AllVars testmod that does not use the bareground index.