Do not allow notifications with custom views in the Conversations section.

We do not have a way to show decorated custom view notifications that match the visual style (especially the icon) of other conversations, and it causes significant user confusion when notifications that do not match the conversation UX are in that (unlabeled) section.

Since we do not want to change how we render the notification (e.g. simply discard the custom view) we abide by the appearance and change the ranking.

Change-Id: I2d7609c925666d0465b15aeca9c15e366a4c69ad
Flag: android.app.notification_no_custom_view_conversations
Bug: 368817201 
Test: validated with notify2