CD updater: add missing call in tests

ag/31885242 changed the behavior for registerCallback such that it no
longer automatically scheduled an update. This CL adds the missing call
in tests.

Flag: com.android.settings.flags.display_topology_pane_in_display_list
Test: atest ExternalDisplayUpdaterTest.java
Test: atest ConnectedDeviceGroupControllerTest.java
Bug: b/399273324
Change-Id: If030e7cc98289fe3062c1a638580ec6c6479d97a