[Notif redesign] Adjust spacing for messaging image

Adjust the spacing for the image shown in collapsed messaging
notifications to match the one for the large icon. Also adjust the
expander padding in the same way we do for the large icon.

Duplicated some logic from MessagingLayout into ConversationLayout for
consistency in hiding the large icon when the messaging icon is showing,
since we additionally need to make sure we avoid messing with the
margins when they've already been adjusted for the large icon.

Fix: 404557681
Test: manual
Flag: android.app.notifications_redesign_templates
Change-Id: I7c78a712be1e5e5a5e9279061d8ca9e305e3eaa7