SeekBarVolumizer: fix media volume initialization for VolumeGroup

The VolumeGroup callback was initializing the sliders by querying
  the VolumeGroup API, which talks to APM. In AVCRP with a
  BT headset that supports absolute volume, the APM only sees the
  media volume at max, as no digital attenuation is applied then,
  which causes the VolumeGroup API to report max volume.
The fix consists in using the updateSlider() method instead, which
  will cause the volume query to go through AudioService (which
  maintains the actual volume setting independently of the digital
  attenuation that is applied).

Bug: 132287865
Test: connect BT headset with abs volume, go to Settings > Sound

Change-Id: I3d46af69d808169806c86cb543440f262097965b