Use system default key press volume

Bug: 9475973
Change-Id: I6ae825b161dde69e19b52d73fb9aa782e2aa7423

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>

java/src/com/android/inputmethod/latin/SettingsFragment.java

@@ -453,6 +453,9 @@ public final class SettingsFragment extends InputMethodSettingsFragment
 
             @Override
             public String getValueText(final int value) {
+                if (value < 0) {
+                    return res.getString(R.string.settings_system_default);
+                }
                 return Integer.toString(value);
             }