Rename Platform::GetFile (3-arg version) to GetFileWithUUID

This rename was suggested by gclayton as a way to silence gcc
warnings; the warning is emitted when there is an overloaded function
in a base class (Platform) for which a derived class redefines one of
the overloads but not the other (because doing so hides the other
overload from users of the derived class).  By giving the two methods
different names, the situation is avoided.