Add no data usage string

Add a no data usage string so we can avoid showing incorrect dates
when no data is available.

Bug: 21076561
Change-Id: I4568cd750fe5f973fed0ff4ffa48412d8c6afd97

res/values/strings.xml

@@ -6696,4 +6696,7 @@
 
     <!-- Label for maximum amount of memory app has used [CHAR LIMIT=25] -->
     <string name="memory_maximum_usage">Maximum usage</string>
+
+    <!-- Summary of data item when no data usage [CHAR LIMIT=40] -->
+    <string name="no_data_usage">No data used</string>
 </resources>

src/com/android/settings/applications/InstalledAppDetails.java

@@ -516,6 +516,9 @@ public class InstalledAppDetails extends AppInfoBase
     private CharSequence getDataSummary() {
         if (mChartData != null) {
             long totalBytes = mChartData.detail.getTotalBytes();
+            if (totalBytes == 0) {
+                return getString(R.string.no_data_usage);
+            }
             Context context = getActivity();
             return getString(R.string.data_summary_format,
                     Formatter.formatFileSize(context, totalBytes),