Don't load usage events into memory for usage statistic queries

Avoid unnecessary IOs and additional memory overhead for usage
statistica queries.

Also dump the pending usage events for unlocked user(s).

Bug: 301449929
Test: atest CtsUsageStatsTestCases
      atest FrameworksCoreTests:android.app.usage.UsageStatsPersistenceTest
      atest FrameworksServicesTests:com.android.server.usage.UsageStatsDatabaseTest
Change-Id: Ia68e78a96dd1f66d19744acfb85b74e890608df7