Use PrefRefresh for Preference caching

Refactor a couple of fields to use PrefRefresh for caching instead
instead of fields. This makes us construct PrefRefresh for display
categories, which we hadn't been doing before, and was causing
b/397466966. This also limits the tests' dependency on implementation.

We had been using fields for caching Preference objects, which doesn't
account for the same preference being needed for each display.
PrefRefresh is a more flexible mechanism for recycling View objects and
it largely makes the fields obsolete.

The reuse* methods (previously get*Preference) can actually be inlined
now that tests aren't calling into them, but to keep the diff easy to
review I left them as separate methods.

Because @NonNull doesn't actually do anything, and is considered the
default, I neglected to add them to new arguments, and deleted them from
lines I modified.

Flag: com.android.settings.flags.display_topology_pane_in_display_list
Test: ExternalDisplayPreferenceFragmentTest.java
Test: with fragment visible, plug-in and activate a new physical display with the Mirror prompt
Test: with topology disabled, verify footer is shown with and without a display attached
Bug: b/397466966
Bug: b/397591022
Change-Id: I16b3ae925ca3d9d79471fd5df00b571ae3fc696f