[media_previews] Fix stable device id for system default

Device ranking uses name as the stable id, but for media types the
name of the system default is an empty string. That caused a mismatch
with blink types where it's set to "Default". This CL adds a special
case for the system default device to ensure that it uses the same
stable ID in all cases.

Bug: 390333516
Change-Id: I11f306379392600f55dd4fcf3d01bc6ecaa4a585
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6180279
Reviewed-by: Ahmed Moussa <ahmedmoussa@google.com>
Commit-Queue: Bryant Chandler <bryantchandler@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1407588}