Audio: Unmute after set new device index during SHO.

Unmute during SHO does not works as it tries to set
mIsMuted flag before new index for bt device is set.

Unmute the flag after new index is set.

CRs-Fixed: 2490722
Change-Id: I6b3c10569a19d565299b9868a8561f4ef289a6d6