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

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.

src/config/main.lua.d/40-device-defaults.lua

@@ -8,7 +8,7 @@ device_defaults.properties = {
   ["use-persistent-storage"] = true,
 
   -- the default volume to apply to ACP device nodes, in the linear scale
-  --["default-volume"] = 0.4,
+  --["default-volume"] = 0.064,
 
   -- Whether to auto-switch to echo cancel sink and source nodes or not
   ["auto-echo-cancel"] = true,

src/scripts/policy-device-routes.lua

@@ -14,7 +14,7 @@ local config = ... or {}
 use_persistent_storage = config["use-persistent-storage"] or false
 
 -- the default volume to apply
-default_volume = tonumber(config["default-volume"] or 0.4)
+default_volume = tonumber(config["default-volume"] or 0.4^3)
 
 -- table of device info
 dev_infos = {}