Fix Talkback focus order of Wrapped Notification

Make notification_header not important for accessibility to make sure early recursion happens in parent's ViewGroup.addChildrenForAccessibility(ArrayList<View> outChildren)

Bug: 246950513
Test: Visual, turn on Talkback, mock a wrapped notification(collapsed), right swipe to move Talkback focus. Expand button should be focused prior to the first line of Wrapped Text.
Change-Id: I98d5c3158b25edf58947e72c5307eae159915dc3