Update headphones volume warning threshold

Update headphones volume warning threshold after
measurements.

Bug: 9280753.

Change-Id: I085e6be0908f84cdc335ddc40c5a16850ea398eb
diff --git a/overlay/frameworks/base/core/res/res/values/config.xml b/overlay/frameworks/base/core/res/res/values/config.xml
index 9add950..c849fcd 100644
--- a/overlay/frameworks/base/core/res/res/values/config.xml
+++ b/overlay/frameworks/base/core/res/res/values/config.xml
@@ -237,7 +237,7 @@
     </string-array>
 
     <!-- reference volume index for music stream to limit headphone volume and display warning -->
-    <integer name="config_safe_media_volume_index">7</integer>
+    <integer name="config_safe_media_volume_index">6</integer>
 
     <bool name="config_enableWifiDisplay">true</bool>