Avoid extra string creation

We're calling a function that takes a const reference to a
std::string.  When passing the result of c_str(), it forces
the creation of a new std::string object.  By directly passing
a const reference our already created std::string, we avoid this.

Test: TreeHugger
Bug: 196432585
Change-Id: I40bc8ebd0cf59452a59ae2a350ddb2542eb5b3b0