Report widget interaction events

This change adds IAppWidgetService.reportWidgetEvent API for reporting
widget interaction events generated by AppWidgetHostView. These events
are validated in the service then reported to UsageStatsManager.

The widget service merges all events within a configurable period and
reports them as one event to UsageStatsManager.

Bug: 364655296
Test: atest CtsAppWidgetTestCases:WidgetEventsTest
Flag: android.appwidget.flags.engagement_metrics

Change-Id: I049d4f4cbe2154c1ede61317add9c4b7b079f799