bluetooth: Add 20dB boost for faststream source volume

Some devices unexpectedly return stereo backchannel stream with wrong frequency,
and volume of backchannel stream appears too low.

If this happens, boost source volume by 20dB.
2 jobs for pr/bluetooth-faststream in 1 minute and 53 seconds (queued for 5 seconds)
latest
Status Name Job ID Coverage
  Container
passed build-container #13578811

00:00:17

 
  Build
passed build-meson #13578812

00:01:36