Do not show entry point when no CD attached

Do not show External Displays entry point unless one (or more) external
displays is actually attached.

Flag: com.android.settings.flags.display_topology_pane_in_display_list
Bug: b/352648432
Test: atest ExternalDisplayPreferenceFragmentTest.java
Test: attach one external display and open CD settings - verify the topology pane is shown
Test: add and remove an external display and verify the connected device entry point is added and removed
Change-Id: If263e98aeaf7be2778c1e8a1cbafc7e2345bb0c5