Improve titles

- makes titles consistent with what is shown into the Drawer and
defined into the PreferenceScreen files

Change-Id: I6077d947b3afcc63bfbe9c78b973709778064f9b

res/xml/power_usage_summary.xml

@@ -15,7 +15,7 @@
 -->
 
 <PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android"
-        android:title="@string/battery_since_unplugged"
+        android:title="@string/power_usage_summary_title"
         android:key="app_list">
     <Preference
         style="?android:attr/preferenceInformationStyle"