Ensure UsageStats data is read correctly.

Add documentation that the IntervalStats object passed into the read
methods in UsageStatsDatabase should ideally be used only as a temporary
reading object unless the caller specifically has other use-cases. Also
updated all the internal calls to match this behavior.

Bug: n/a
Test: atest UsageStatsDatabaseTest
Change-Id: Id3aa2575fa29bf5adc6a52ee8e75690aecf50b8e