Tune keepalive timeouts for Tensorflow/GRPC

This disables the keepalive watchdog for TF/GRPC channels.  The watchdog ping timer is intended to monitor channels in case they have gone "stale".  If this occurs, any pending RPCs are marked failed.  This interacts poorly with large TF models, where we can saturate the network exchanging tensors, causing the watchdog ping to be delayed.

The timer is not essential (normal deadline processing and socket termination is still respected), so we can disable it with minimal risk here.

PiperOrigin-RevId: 224913045