Add use of PendingIntent.getForegroundService()

In particular, we have an example of a foreground-service PendingIntent
triggered by a 15-second delayed alarm following a button click.  This
is enough time to exercise use cases like swiping the app out of recents
before the alarm fires, therefore entering the "starting a fg service
when the app is not in a fg state" flows.

Bug: 73559697
Test: this is it
Change-Id: If57908ab852c7a0bfab7bb1da8eda0a47493fc6b