audio: Fix to enable USB volume during music playback

- The USB device had been added to the mFixedVolumeDevices
  because of which volume change was disabled for music
  stream
- Fixed by removing the USB device from the
  mFixedVolumeDevices.

Change-Id: I82b1876d152030ddcfe9ffc31924b6abe4b2509a
CRs-fixed: 439858
(cherry picked from commit b7ed482f7654d5657fb7497a95cc007229478886)
(cherry picked from commit ae67047a3f95ddde2565a76c44254ad714d3109d)