Move hybrid notification view inflation to the background thread

Move the hybrid notification view inflation, including icon loading
to the background thread to reduce janks and improve the performance of
System UI. This change refactors the code structure to decouple the
inflation of single-line views and the ConversationLayout RemoteViews
creation.

Bug: 217799515
Test: atest NotificationContentInflaterTest
Flag: ACONFIG notification_async_hybrid_view_inflation DEVELOPMENT

Change-Id: If1eba1bb1d66e1231f2337cd8ca023ebf0f45211