Fix free page calculations on iOS

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

Libraries/Image/RCTUIImageViewAnimated.m

@@ -31,7 +31,7 @@ static NSUInteger RCTDeviceFreeMemory() {
   if (kern != KERN_SUCCESS) return 0;
   kern = host_statistics(host_port, HOST_VM_INFO, (host_info_t)&vm_stat, &host_size);
   if (kern != KERN_SUCCESS) return 0;
-  return vm_stat.free_count * page_size;
+  return (vm_stat.free_count - vm_stat.speculative_count) * page_size;
 }
 
 @interface RCTUIImageViewAnimated () <CALayerDelegate>