Fix use of getAsI when parsing download rpc.

The value is store as a string in in the xml, so we cannot use getAsI.
We have to get the string and parse it to an int.
We cannot use strtoull because android stdc++ lib doesn't have it.

We have to implement our how parseFromString function using a
istringstream.

include/common/stringTools.h

@@ -71,5 +71,13 @@ std::string to_string(T value)
     oss << value;
     return oss.str();
 }
+
+template<typename T>
+T extractFromString(const std::string& str) {
+    std::istringstream iss(str);
+    T ret;
+    iss >> ret;
+    return ret;
+}
 } //namespace kiwix
 #endif