alsa_sound: Flush Proxy data at high water mark

-There are instances when delays in the system setup
causes buffering to fill up in Proxy. This results
in Proxy buffers being very close to full leading
to choppy audio
-To fix this we need to flush the data in the proxy
whenever the high water mark is reached. The water
mark is set to 40000 frames for now. This can be
changed by changing the
AFE_PROXY_HIGH_WATER_MARK_FRAME_COUNT

Change-Id: Ie727ac07c4865c8e84f1dfda752bc4b17b84a926