siso: disable keepalive on unix domain socket

keepalive is needed to detect zombie transport connections,
but unix domain socket operates in local kernel memory
on the same host, so disconnect are signaled immediately
by the OS via EOF/POLLHUP.
To prevent false-positive PING ACK timeouts under high local load,
disable keepalive on unix domain socket.

Bug: b/537158784
Change-Id: I8e5e694e7e7d7861bdfc858eab07cc81a2b65866
Reviewed-on: https://chromium-review.googlesource.com/c/build/+/8150839
Commit-Queue: Junji Watanabe <jwata@google.com>
Auto-Submit: Fumitoshi Ukai <ukai@google.com>
Reviewed-by: Junji Watanabe <jwata@google.com>