audio HAL : added string for A2DP address

Added a definition for A2DP sink address parameter.
It was previously defined at several places.

Change-Id: I757208e99db395f3d88c17e0de5ae303c71a85ec
diff --git a/include/hardware/audio.h b/include/hardware/audio.h
index 1e68cd1..af4df04 100644
--- a/include/hardware/audio.h
+++ b/include/hardware/audio.h
@@ -63,6 +63,9 @@
 #define AUDIO_PARAMETER_VALUE_TTY_HCO "tty_hco"
 #define AUDIO_PARAMETER_VALUE_TTY_FULL "tty_full"
 
+/* A2DP sink address set by framework */
+#define AUDIO_PARAMETER_A2DP_SINK_ADDRESS "a2dp_sink_address"
+
 /**
  *  audio stream parameters
  */