AudioService: fix redundant stream volume init

When applying volume from a VolumeGroupState,
skip stream volume update if this stream is already managed by a
VolumeStreamState.

Test: set Music volume & reboot & dumpsys audio
    Ensure indexes matches
Signed-off-by: Francois Gaffie <francois.gaffie@renault.com>
Change-Id: If80f6324865d1fb076cc4de3ee72591e4653bf74