CD settings: minimal DisplayDevice metadata class

This saves tests from interacting with DisplayManager completely,
simplifies the Injector API, and makes mocking that API easier. This
addresses the first difficulty I had in converting our tests to
Robolectric.

Bug: b/399743032
Flag: com.android.settings.flags.display_topology_pane_in_display_list
Test: verify disabled and enabled displays appear correctly in display list fragment
Test: atest ExternalDisplayPreferenceFragmentTest.java
Test: atest ExternalDisplayUpdaterTest.java
Test: atest ResolutionPreferenceFragmentTest.java
Change-Id: Id43d2f108f3e85e6596eb2271b1de6b1afd2338f