Updates to widget impressions for review feedback

- AppWidgetEvent.Builder uses longs and System.currentTimeMillis to
  avoid allocating temporary objects
- AppWidgetServiceImpl uses JobScheduler instead of AlarmManager for
  periodic event reporting jobs
- Remove @FlaggedApi annotation from hidden APIs

Bug: 425685745
Bug: 375515125
Test: FrameworksCoreTests:AppWidgetEventsTest CtsAppWidgetTestCases:WidgetEventsTest
Flag: android.appwidget.flags.engagement_metrics
Change-Id: I87ad16f5f7eadba3bc83a4df47f38ccbe68b3a84