Audio: Unmute before set device volume during SHO.

No audio heard after SHO when one remote volume set to zero.

When one remote volume is set to zero the stream becomes inactive
and flag mIsMuted becomes true.Now when Soft hand off happens and
A2dp sends us active device change with new remembered a2dp volume
we dont reset the mIsMuted flag and applyDeviceVolume such that if
mIsMuted is true index is overwritten to true.

To solve this issue we reset the mute flag before setting device volume
so that index does not gets overwritten and correct index gets set.

CRs-Fixed: 2452290
Change-Id: Id9d09e61067c4650a0e0575e3a1b96f2dd6e6b2c