Actually listen to data saver state

Bug: 27445032
Change-Id: Idb9646af9cd82e19b3d4a6add52d349be8015caa
diff --git a/src/com/android/settings/datausage/DataSaverSummary.java b/src/com/android/settings/datausage/DataSaverSummary.java
index e8fb2fc..dc8f125 100644
--- a/src/com/android/settings/datausage/DataSaverSummary.java
+++ b/src/com/android/settings/datausage/DataSaverSummary.java
@@ -70,6 +70,7 @@
     public void onResume() {
         super.onResume();
         mDataSaverBackend.refreshWhitelist();
+        mDataSaverBackend.addListener(this);
         mSession.resume();
         mDataUsageBridge.resume();
     }
@@ -77,6 +78,7 @@
     @Override
     public void onPause() {
         super.onPause();
+        mDataSaverBackend.remListener(this);
         mDataUsageBridge.pause();
         mSession.pause();
     }