Revert "Pass std::strings through the OStreamWrapper."

This reverts commit c49b9c19.

I'm going to remove this commit until we can figure out what to do
with strings. The conventions say std::string is our preferred string
container, but also that STL containers cannot be used in Common/Core.

Change-Id: I1db2bad22bb74ca19de5355fb44371b58b59e900