Store last-resume times in a separate XML file.

In UsageStatsService, separate last-resume times from the rest of
the statistics, and serialize them to an XML file daily.

This way, ApplicationsProvider will still be able to acces this data,
even thoguh other statistics are flushed to disk and discarded each day.

Bug: 5108745
Change-Id: Id3df3c98243ba02cde16b31e5e29bd9ff3602108
diff --git a/services/java/com/android/server/am/ActivityManagerService.java b/services/java/com/android/server/am/ActivityManagerService.java
index d3e12f7..d5a1b8f 100644
--- a/services/java/com/android/server/am/ActivityManagerService.java
+++ b/services/java/com/android/server/am/ActivityManagerService.java
@@ -5847,6 +5847,8 @@
         }
 
         mSelf.mCoreSettingsObserver = new CoreSettingsObserver(mSelf);
+
+        mSelf.mUsageStatsService.monitorPackages();
     }
 
     /**