hal: fix BT device switch issue for audio playback during VoIP call

- When VoIP call is active on non-BT device and if the music playback
tries to switch to BT device, it will continue to route to non-BT
device on which the VoIP call is active already.
- Fix the issue by not selecting VoIP devices for audio playback when
they are on different back ends.

Change-Id: Iff9251fcf02f897d9b1cdc17fcf27c262518ada8