audio HAL: add defines for BT NREC parameter

Added definitions of keys and values used to enable or disable
AEC and NS via set_parameters according to BT headset capabilities.

Change-Id: Id3fba06edfe77e2832b8256c0c4c46eeef17b816
diff --git a/include/hardware/audio.h b/include/hardware/audio.h
index 36ff399..75c5773 100644
--- a/include/hardware/audio.h
+++ b/include/hardware/audio.h
@@ -43,6 +43,11 @@
 
 /**************************************/
 
+/* BT SCO Noise Reduction + Echo Cancellation parameters */
+#define AUDIO_PARAMETER_KEY_BT_NREC "bt_headset_nrec"
+#define AUDIO_PARAMETER_VALUE_ON "on"
+#define AUDIO_PARAMETER_VALUE_OFF "off"
+
 /* standard audio parameters that the HAL may need to handle */
 #define AUDIO_PARAMETER_STREAM_ROUTING "routing"
 #define AUDIO_PARAMETER_STREAM_FORMAT "format"