audio: fix stream type for accessibility usage

Make sure that accessibility prompts are heard when
a ringtone is active by forcing stream type
to AUDIO_STREAM_RING when phone state is AUDIO_MODE_RINGTONE.

Bug: 17558149.
Change-Id: Ia3bead8052fca5cbf282c267f7b9b06014fef628
diff --git a/media/libmedia/AudioTrack.cpp b/media/libmedia/AudioTrack.cpp
index ea7b279..e3beba5 100644
--- a/media/libmedia/AudioTrack.cpp
+++ b/media/libmedia/AudioTrack.cpp
@@ -2124,9 +2124,16 @@
 
     // usage to stream type mapping
     switch (aa.usage) {
+    case AUDIO_USAGE_ASSISTANCE_ACCESSIBILITY:
+        // TODO once AudioPolicyManager fully supports audio_attributes_t,
+        //   remove stream change based on phone state
+        if (AudioSystem::getPhoneState() == AUDIO_MODE_RINGTONE) {
+            mStreamType = AUDIO_STREAM_RING;
+            break;
+        }
+        /// FALL THROUGH
     case AUDIO_USAGE_MEDIA:
     case AUDIO_USAGE_GAME:
-    case AUDIO_USAGE_ASSISTANCE_ACCESSIBILITY:
     case AUDIO_USAGE_ASSISTANCE_NAVIGATION_GUIDANCE:
         mStreamType = AUDIO_STREAM_MUSIC;
         return;