hal: avoid redundant device switch during playback.

With music playback in background, a new playback is started, current 
snd_device of this new playback usecase is none at this moment, 
then force_routing flag is set to true, which causes device 
switch even if output snd devices are same.

Fix it by checking current snd_device to avoid redundant device switch.

CRs-Fixed: 2273928 
Change-Id: Ic4592123bf0d1d9c1349b61fea2af56cf16cabf6