alsa_sound: Fix for AV Sync related issues

-Fix buffer pointer mismatch in proxy
-There were glitches heard at times when listening
to music over Bluetooth
-The issue was due to slight scheduling delays. When proxy read
thread in the HAL wakes up, there might be more than one period
size worth of data in the proxy. This would result in the proxy
copying only one period but incrementing whatever is available and
subsequently leading to glitches.

-Increase extOutThread priority to URGENT_AUDIO to avoid
delays in proxy port read.

-query BT hal for audio latency for a2dpstream
-Set proper latency when audio is played on BT devices
by querying BT hal. This is needed for av sync over BT devices

-Calculate proxy port latency
-The latency of proxy port should be considered during playback
on these devices.

-Flush Proxy data at high water mark
-Delays in the system setup causes Proxy buffers to become very close
to full and leading to choppy audio, afterwards.
-Flush the data in the proxy whenever a limit [water mark] is
reached. The water mark is set to 40000 frames. This can, also, be
changed by modifying AFE_PROXY_HIGH_WATER_MARK_FRAME_COUNT

Change-Id: I55ffb93b16618d3e63fb93c66483286db058740f