hal: mute stream and switch to speaker during a2dp suspend

During A2DP to SCO transition, in order to avoid audible
PCM data pumping to BT SoC during suspend, mute stream
from DSP and switch to speaker to tear down slimbus to
avoid glitches heard after SCO connection is established.

Change-Id: Iabe2216e574b462cfc179369e75007998a576480