Split up DisplayTopology.kt into 1-per-class files

This avoids a file naming conflict with DisplayTopology.java in
DisplayManager, and splits a large file. It was trivial to do since the
file had 3 top-level classes.

Test: local build and SQ
Test: atest DisplayTopologyPreferenceTest.kt
Flag: com.android.settings.flags.display_topology_pane_in_display_list
Bug: b/352648432
Change-Id: I4adc8167ab01b39a6da49f95f0cd072acec67ad4