Revert "audio: Remove unused screen_state parameter"

We want that to avoid an error message in the log, so revert the
revomal.

This reverts commit cdad20da45269c9c5d7308f7d63de1350e04d94d.

Change-Id: Ic90cd6b02e5b3e8aae73d824153c3084643a8acc
diff --git a/audio/audio_hw.h b/audio/audio_hw.h
index 96635ec..8a507ed 100644
--- a/audio/audio_hw.h
+++ b/audio/audio_hw.h
@@ -384,6 +384,7 @@
     struct stream_in*       active_input;
     struct stream_out*      primary_output;
     bool                    mic_mute;
+    bool                    screen_off;
 
     struct voice_data       voice;