[XLA:CPU]  Moving oneDNN threadpool wrapper to tsl 
> Thank you very much for the PR, @agramesh1! The overall PR looks good to me. I have two minor comments.

@penpornk I made the changes. Please take a look at it. 
This PR moves the oneDNN threadpool wrapper code under tsl, and uses tsl namesapce so it can also be called from XLA code.  
Changes include
1)	Removed dependency on reading an TF environment flag for using calling thread. The call  for reading the flag  (ThreadPoolUseCallerThread()) is removed and the env value is now passed into the constructor from TensorFlow.
2)	Pointer to the Eigen threadpool is directly passed to the constructor instead of getting it from OptKernelContext.
3)	Some additional changes for clarity and using oneDNN instead of mkl in variables and filenames.
