Avoid accurately computing disk space on iOS and Android (where it can be slow). Add a setting.

ios/main.mm

@@ -415,6 +415,9 @@ bool System_GetPropertyBool(SystemProperty prop) {
 		case SYSPROP_CAN_RESTRICT_ORIENTATION:
 			return true;
 
+		case SYSPROP_CAN_GET_FREE_SPACE_FAST:
+			return false;
+
 		default:
 			return false;
 	}