2/n ConversationLayout: offload conversation icon loading to background thread

This CL offloads Conversation Avatar drawable loading to the background thread. Conversation header data extraction logic including drawable loading is extracted to loadConversationHeaderData method.
It is used in setDataAsync when `conversation_style_set_avatar_async` is enabled.

Bug: 305540309
Test: presubmit
Flag: ACONFIG android.widget.flags.conversation_style_set_avatar_async DEVELOPMENT
Change-Id: Ib13c76d500073adcb0fb0cb183507bec4a1e8e06