Prevent Conversations from using custom views.

Conversations with custom views do not look like conversations, and mess with the consistency of the shade and cause users confusion about why a totally normal-looking notification marked as silent is not int he silent section of the shade.

Flag: android.app.notification_no_custom_view_conversations
Fixes: 368817201
Test: validated with culprit app's notification
Change-Id: Id34a532e04c03cd4a1af36c205fb72356312f72f