dcsctp: Resend SHUTDOWN ACK on timer expiration

If the T2-shutdown timer expires while the receiver is in the
SHUTDOWN-ACK-SENT state, it MUST resend the SHUTDOWN ACK chunk.

Previously, OnShutdownTimerExpiry would always call SendShutdown(),
meaning that if the timer expired in the SHUTDOWN-ACK-SENT state, it
would incorrectly send a SHUTDOWN chunk instead of a SHUTDOWN ACK.
This CL fixes this by checking the socket state in
OnShutdownTimerExpiry.

Bug: webrtc:500087337
Change-Id: I603c415a162bf7d194ef06a9c5e74dbf3b0dafe8
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/461920
Commit-Queue: Victor Boivie <boivie@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47337}