Unhiding APIs in UsageEvents for public use

These apis are needed to interpret standby bucket change history
when using UsageStatsManager#queryEventsForSelf

Test: atest android.app.usage.cts.UsageStatsTest#testQueryEventsForSelf

Bug: 71906213
Change-Id: Ib9f458bf32089ad30c97eca9dd5cc4df5a392191