Fix issue 2192181: AudioFlinger must provide separated methods to set VOICE_CALL stream volume and down link audio volume.

Added setVoiceVolume() method to AudioSystem, AudioFlinger, IAudioFlinger, AudioPolicyService.
Removed call to AudioHardwareInterface::setVoiceVolume() from AudioFlinger::setStreamVolume().
diff --git a/media/libmedia/AudioSystem.cpp b/media/libmedia/AudioSystem.cpp
index bd1b2d7..5352234 100644
--- a/media/libmedia/AudioSystem.cpp
+++ b/media/libmedia/AudioSystem.cpp
@@ -335,6 +335,13 @@
     return NO_ERROR;
 }
 
+status_t AudioSystem::setVoiceVolume(float value)
+{
+    const sp<IAudioFlinger>& af = AudioSystem::get_audio_flinger();
+    if (af == 0) return PERMISSION_DENIED;
+    return af->setVoiceVolume(value);
+}
+
 // ---------------------------------------------------------------------------
 
 void AudioSystem::AudioFlingerClient::binderDied(const wp<IBinder>& who) {