audio: reset output device when disconnecting HDMI

When HDMI is disconnected, the audio HAL should stop writing to
the PCM driver to avoid a write error and a long timeout.
THe problem is that if no audio track is active, the policy manager
will send a 0 device upon HDMI disconnection which is normally ignored.

The fix consists in forcing the device to speaker when transitioning from
HDMI to 0 device.

Bug 7141149.

Change-Id: I3e5878a1d6ec0446f7044eff95e6641332c718bf