Make pre-scale for absolute volume configurable

Audio gain for lower volume steps are restricted for
Bluetooth Absolute Volume scenario, but it's better
not to use the fixed value.
This CL makes it possible to configure the value of
pre-scale.

Bug: 114220617
Test: manual - gain is restricted as per the configuration

Change-Id: I1fd0c77476386ba9518e1819d5ea2c7b0c344a40