Settings: Hide phone-related settings on tablets.

Bug:15002393
Bug:15091008
Change-Id: I8238b7990572f46e1530a800897ab2d39a189055
diff --git a/res/xml/notification_settings.xml b/res/xml/notification_settings.xml
index 633c133..865138f 100644
--- a/res/xml/notification_settings.xml
+++ b/res/xml/notification_settings.xml
@@ -37,6 +37,12 @@
             android:icon="@*android:drawable/ic_audio_ring_notif"
             android:title="@string/ring_volume_option_title" />
 
+    <!-- Notification volume -->
+    <com.android.settings.notification.VolumeSeekBarPreference
+            android:key="notification_volume"
+            android:icon="@*android:drawable/ic_audio_ring_notif"
+            android:title="@string/notification_volume_option_title" />
+
     <!-- Do not disturb -->
     <PreferenceScreen
             android:key="zen_mode"