Change the default backup connection ping interval to 25 seconds.
Change the default backup connection ping interval to 25 seconds.

This avoids the issue that the backup connection may be pinged faster
than the stable rate (once every 2.5 second) we have chosen for
non-backup connections.

R=deadbeef@webrtc.org, pthatcher@webrtc.org, zhihuang@webrtc.org

Review URL: https://codereview.webrtc.org/2239423002 .

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#13787}

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