Connected displays: use Robolectric for tests

Use Robolectric for tests so that we don't have to deploy to a device
emulator. TestHandler is not available in Robolectric tests, so this CL
includes a substitute.

Flag: com.android.settings.flags.display_topology_pane_in_display_list
Test: atest <all moved tests>
Bug: b/399743032
Change-Id: I95721d9b90376374da7011e42c70e474b494e992