policy-device-routes: change default volume to 0.4 on cubic scale

P V requested to merge pvir/wireplumber:default-volume into master

The default volume level of 74% on Pulseaudio slider cubic scale used by Wireplumber feels a bit high, especially on Bluetooth devices where this value is uncomfortably loud on all devices I have.

It's also applied to input direction, where 100% might be a better default. So it's maybe useful to reconsider these defaults -- here's another suggestion.

It's safer to have the default volume value at significant attenuation, to mitigate situations where the output is at high gain.

Use linear scale 0.4^3 (-24 dB), which puts Pulseaudio volume sliders at 40%, as a better starting value than 0.4 (-8 dB / 74%). E.g. for many Bluetooth headphones, the volume at 74% is still uncomfortably high, and a lower value is better.

policy-default-routes: use different default volume for input direction

Input direction volume doesn't need a safety margin.

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