Fix the seeding for `Dataset.shuffle(..., reshuffle_each_iteration=False)`.

Previously, we were passing the first (graph-level) seed for both the
graph-level and op-level seeds when creating a C++ dataset. This
change passes the op-level seed to the appropriate point, and adds a test
for the behavior with graph-but-not-op-level seeds.

PiperOrigin-RevId: 216280641