Hydrate and test previewIcons

Hydrate previewIcons through Activatable.

Make sure that appIcons are fetched lazily (test added).

Bug: b/416131251 b/416132260 b/416277572
Test: New test added
Flag: com.android.systemui.notification_bundle_ui
Change-Id: I490411b50edc7931e74fb236d22b181117af8496