Added a new eager C API TFE_NewContextFromSession(), where TFE_NewContext will

get an owned device mgr from the input session.

One use case is in S4TF, we run a graph session to enqueue a tensor into a fifo
queue, and then call TFE_Execute() on a dequeue op over the same queue, as a way
to transfer a tensor from TF to host (tensor tranfer in the other direction also
works).

To make this work, we need TFE_Context and the the TF_Session to use the same
ResourceMgr object (attached to a Device, which is in turn owned by DeviceMgr),
so that both can access the fifo queue resource op.

PiperOrigin-RevId: 211471075