Add tests to verify UsageStats persistence.

These new tests verify that all fields defined in UsageStats,
UsageEvents.Event and IntervalStats are known fields. This is to ensure
that when new fields are added to these objects or a refactoring is
done, the fields are accounted for when persisting usage stats.

Bug: 155935953
Test: atest UsageStatsPersistenceTest
Test: atest IntervalStatsTest
Change-Id: Ice65391d0af08b64db4ccaa046e57d9fca299319