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/include/media/AudioSystem.h b/include/media/AudioSystem.h
index 57f8102..a7c8a0a 100644
--- a/include/media/AudioSystem.h
+++ b/include/media/AudioSystem.h
@@ -220,6 +220,7 @@
     static status_t getInputBufferSize(uint32_t sampleRate, int format, int channelCount,
         size_t* buffSize);
 
+    static status_t setVoiceVolume(float volume);
 
     //
     // AudioPolicyService interface