Fix help & feedback link in data usage summary screen.

The fragment was not providing the correct string for help & feedback
button, causing no result page when user navigates to help article page
from actionbar overflow menu.

Change-Id: I076846c44d4fc95efea743f019a7ad4aab47f214
Fixes: 32291069
Test: manual

src/com/android/settings/datausage/DataUsageSummary.java

@@ -76,6 +76,11 @@ public class DataUsageSummary extends DataUsageBase implements Indexable, DataUs
     private NetworkTemplate mDefaultTemplate;
     private int mDataUsageTemplate;
 
+    @Override
+    protected int getHelpResource() {
+        return R.string.help_url_data_usage;
+    }
+
     @Override
     public void onCreate(Bundle icicle) {
         super.onCreate(icicle);