Show a category for each external display

Rather than show all external displays as a single item each under one
category, each external display has its own category. For now, the
category only has one item: the single Resolution+Rotation option.

This is more consistent with the Built-in display, which also still
only has one preference item in the category.

Flag: com.android.settings.flags.display_topology_pane_in_display_list
Bug: b/352648432
Bug: b/392946322
Test: ExternalDisplayPreferenceFragmentTest
Test: Open display list with two displays connected, and remove them one at a time
Change-Id: I4d7f8410f506fe53201bc87f8b55ffc6d407a9c5