Summary:
[iOS] [Fixed] - Fix how the amount of free memory is calculated to mimic the logic Apple uses.
For example, see https://opensource.apple.com/source/system_cmds/system_cmds-805.250.2/vm_stat.tproj/vm_stat.c.auto.html for how `vm_stat` does it:
```
sspstat("Pages free:", (uint64_t) (vm_stat.free_count - vm_stat.speculative_count));
```
Reviewed By: shergin
Differential Revision: D17671714
fbshipit-source-id: 18ef31e17a0527a9bef7a408922cd687260866db