Allow users to set notification topic importance.

In the absence of mocks, the silder is shown in the
'normal' position if the user hasn't yet
chosen an importance value.

Bug: 22854014
Change-Id: I51594959412775fe89b29af66ddcb13bafa67255
diff --git a/res/xml/topic_notification_settings.xml b/res/xml/topic_notification_settings.xml
index 6826070..174c273 100644
--- a/res/xml/topic_notification_settings.xml
+++ b/res/xml/topic_notification_settings.xml
@@ -18,6 +18,13 @@
         android:title="@string/topic_notifications_title"
         android:key="topic_notification_settings">
 
+    <!-- Importance -->
+    <com.android.settings.notification.ImportanceSeekBarPreference
+            android:key="importance"
+            android:title="@string/app_notification_importance_title"
+            android:order="1"
+            android:persistent="false" />
+
     <!-- Bypass DND -->
     <SwitchPreference
             android:key="bypass_dnd"