add vibration duration settings

Change-Id: I56a1a94df63160f367f866da759d7dd4fc428ed5
diff --git a/java/res/xml/prefs.xml b/java/res/xml/prefs.xml
index 24de95f..312af28 100644
--- a/java/res/xml/prefs.xml
+++ b/java/res/xml/prefs.xml
@@ -125,6 +125,9 @@
                 android:summary="@string/enable_span_insert_summary"
                 android:persistent="true"
                 android:defaultValue="true" />
+            <PreferenceScreen
+                android:key="pref_vibration_duration_settings"
+                android:title="@string/prefs_vibration_duration_settings"/>
             <!-- TODO: evaluate results and revive this option. The code
                 already supports it. -->
             <!-- <CheckBoxPreference -->