Move cold notification flows to the bg thread.

Bug: 293167744
Bug: 278765923
Flag: ACONFIG com.android.systemui.notifications_footer_view_refactor DEVELOPMENT
Flag: ACONFIG com.android.systemui.notifications_icon_container_refactor DEVELOPMENT
Test: existing tests should still pass, checked some basic functionality
to make sure it looks fine

Change-Id: I4ea40c3a59f8969274f46d96de9526155cb3c4f8
6 files changed