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/services/audiopolicy/AudioPolicyService.cpp b/services/audiopolicy/AudioPolicyService.cpp
index 50bb8c7..647cda4 100644
--- a/services/audiopolicy/AudioPolicyService.cpp
+++ b/services/audiopolicy/AudioPolicyService.cpp
@@ -59,7 +59,7 @@
 
 AudioPolicyService::AudioPolicyService()
     : BnAudioPolicyService(), mpAudioPolicyDev(NULL), mpAudioPolicy(NULL),
-      mAudioPolicyManager(NULL), mAudioPolicyClient(NULL)
+      mAudioPolicyManager(NULL), mAudioPolicyClient(NULL), mPhoneState(AUDIO_MODE_INVALID)
 {
     char value[PROPERTY_VALUE_MAX];
     const struct hw_module_t *module;