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/include/media/AudioSystem.h b/include/media/AudioSystem.h
index dd63a23..f8c0198 100644
--- a/include/media/AudioSystem.h
+++ b/include/media/AudioSystem.h
@@ -320,6 +320,8 @@
                                            audio_devices_t *device);
     static status_t releaseSoundTriggerSession(audio_session_t session);
 
+    static audio_mode_t getPhoneState();
+
     // ----------------------------------------------------------------------------
 
     class AudioPortCallback : public RefBase