Communicate selection state to a11y services.

DocumentsUI uses View#setActivated instead of View#setSelected for its
selction states, because setActivated propagates downward in the view
hierarchy (i.e. updates the state on its child views), while setSelected
does not. a11y services uses View#isSelected to communication selection
state. Thus, we attach the information to the accessibilityNodeInfo for
each RecyclerView item.

Bug: 35322527
Change-Id: I734fe7c00cfc8f2e57eadb519a09067097191d7a