Check for app widget ID extra

Sometimes widget events did not have all of their extras saved. This was
fixed by ag/35480657, but there may still be old events that did not
have their extras saved. This change updates isAppWidgetEvent so that
events without at least an app widget ID extra are excluded.

Bug: 440630000
Test: CtsAppWidgetTestCases:WidgetEventsTest
Flag: android.appwidget.flags.engagement_metrics
Change-Id: Ia2eb9cb3dc81d0d5a01ca953a166588dd24c9d5d