dexopt: Avoid a couple std::string copies

There are a couple places where we're taking a std::string and
then passing the result of c_str() to function taking a
const std::string reference.  This inefficiently creates a
temporary object.  We change these to directly pass the
std::string.

Test: TreeHugger
Change-Id: Ied6f13e5e2b7b71c992d325ce285fd829bdb7ee9