[XLA] Clone EigenThreadPoolWrapper into backend.cc rather than including it.

This is the only library in XLA that depends on the TF common_runtime/ library. (There are some binaries and tests that also use common_runtime/, but this is a useful dependency to break since it means that XLA users don't take an unnecessary TF dependency.) The class is trivial (a adapter between Eigen's thread pool interface and a tensorflow::ThreadPool), so we might as well clone it rather than take the dependency.

Do some renaming so we don't have two things named EigenThreadPoolWrapper.

PiperOrigin-RevId: 234481978