audio: fix crashes upon USB device connection

- Audio policy:
Replace unknown device assertion by debug log in setDeviceConnectionState().
It is possible that some kernels indicate connection of a legacy device type
when a USB audio device is connected.
We should just ignore this device.

- Audio flinger:
Make sure FastMixer thread is idle before exiting the normal mixer thread and
closing the output stream.

Change-Id: Ia10a20176a60a8aa56765538093a555fc998508a