Add constants for app widget usage events

This change adds constants which define the data that is available
within app widget USER_INTERACTION events. It also defines a new
resource ID to be used to set metrics tags for RemoteViews layouts.
This change also adds helper functions for creating event bundles using
these constants.

Adds AppWidgetEventsTest in coretests.

Bug: 364655296
Test: atest FrameworksCoreTests:AppWidgetEventsTest
Flag: android.appwidget.flags.engagement_metrics
Ignore-AOSP-First: folder is new in this branch
Change-Id: Ib482600f55d99a8e817fd901cae715fadadb57d9