audio: add usage "virtual source" to audio attributes.

Bug: 16006090.
Change-Id: I2d22b52e97095b57a6508b82bd11e27f860bb2e7
diff --git a/include/system/audio.h b/include/system/audio.h
index 2749ce0..181a171 100644
--- a/include/system/audio.h
+++ b/include/system/audio.h
@@ -100,6 +100,7 @@
     AUDIO_USAGE_ASSISTANCE_NAVIGATION_GUIDANCE     = 12,
     AUDIO_USAGE_ASSISTANCE_SONIFICATION            = 13,
     AUDIO_USAGE_GAME                               = 14,
+    AUDIO_USAGE_VIRTUAL_SOURCE                     = 15,
 
     AUDIO_USAGE_CNT,
     AUDIO_USAGE_MAX                                = AUDIO_USAGE_CNT - 1,