[SingleLineConversationView] Align avatar logic with ConversationLayout

While conversationIcon's default is shortcut icon in ConversationLayout, it is not for SingleLineView. That causes inconsistency bw existing ConversationLayout and our new hybrid views.

Fixes: 343213166
Test: Check Messenger group chat icon + check group icon by using Notify
Flag: com.android.systemui.notification_async_hybrid_view_inflation
Change-Id: I047075126d4c5e81cb67073e918f0f4a456a9f4b