Use system default key press volume

Bug: 9475973
Change-Id: I6ae825b161dde69e19b52d73fb9aa782e2aa7423
diff --git a/java/res/values/keypress-volumes.xml b/java/res/values/keypress-volumes.xml
index a096c34..d359055 100644
--- a/java/res/values/keypress-volumes.xml
+++ b/java/res/values/keypress-volumes.xml
@@ -26,7 +26,7 @@
         <item>HARDWARE=grouper,0.3f</item>
         <item>HARDWARE=mako,0.3f</item>
         <item>HARDWARE=manta,0.2f</item>
-        <!-- Default value for unknown device -->
-        <item>,0.2f</item>
+        <!-- Default value for unknown device. The negative value means system default. -->
+        <item>,-1.0f</item>
     </string-array>
 </resources>