Fix classified children showing as heads up

This issue happened because summaries
- rarely have text
- will rarely be classified
- can still transfer heads up behavior to silent classified children

In HeadsUpCoordinator's onBeforeFinalizeFilter,
do not transfer alerts to classified children,
which are silent and should not heads up.

Fixes: 401101330
Test: HeadsUpCoordinatorTest
Flag: android.service.notification.notification_classification
Change-Id: If6cffe2b21d62c0832efac375cc1ebb26af029f8