[Conversation Notification] Fix Hybrid Conversation Avatar Loading

HybridConversationNotificationView uses Avatar information of ConversationLayout during binding. When async avatar loading is enabled, icon information is not available but drawable.
This CL fixes that issue by covering drawable case.

Bug: 305540309
Fixes: 330494691
Test: Choose Messaging Style Notifications and Click Add Group in Notify. Observe images are loaded correctly
Flag: ACONFIG android.widget.flags.conversation_style_set_avatar_async TEAMFOOD
Change-Id: I46c328361e117a27ab94a0e98325cfbe23532a98