alsa-mixer: add mixer handling to the fallback stereo case

alsa-mixer: add mixer handling to the fallback stereo case

Some sound cards don't have any alsa-lib configuration, but they used to
work well enough up to PulseAudio 10. PulseAudio 11 stopped using "hw:0"
for the analog-stereo mapping, and instead defined it as a fallback
mapping without any mixer handling. As a result, switching between
headphones and speakers stopped working without changing the mixer
settings manually at least on Toshiba Chromebook 2. This patch adds the
mixer handling back to the fallback mapping.

I also renamed "unknown-stereo" to "stereo-fallback", because I like
that name more.

BugLink: https://bugs.freedesktop.org/show_bug.cgi?id=102560