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