Make system dialogs' a11y events like Activity's

The window_state_changed events are populated by
Activity with the Activity title. Replicating that
behavior for volume and power dialogs.

Bug: 78775496
Test: Used a test a11y service to observe events,
and started TalkBack, which was less consistent
about announcing them.

Change-Id: Icd4cd8317544c127e44e403162512e2385ff4176