Fix broken dark ui test

It was checking something in shared preferences which seems
to have stopped working (a bunch of the stuff used in the
test is deprecated). This CL just has it verify that
updateDarkTheme() is called when appropriate since that
is all we really care about.

Test: is a test
Bug: 130897882
Change-Id: If3286dd91b5a95cb0f41cac619a11a9089e0aecf