Show built-in display if topology is visible

The topology in the External Displays list will include the built-in
display. For consistency, also show the built-in display along with the
connected displays, so there is a 1:1 correspondence between blocks in
the topology pane and display preference items.

Test: launch external display fragment with zero or one external display, verify built-in display item appears and opens Display settings if tapped
Flag: com.android.settings.flags.display_topology_pane_in_display_list
Bug: b/352648432
Change-Id: Id067034f6af5d0d6fe32f4a8091bc77d1ed75698