Clean up SystemUpdatePreferences tests.

SystemUpdatePrefenceControllerTest had some logic that seemed irrelevant
to its testing class, which seems to have moved to
AdditionalSystemUpdatePreferenceController. Removed the irrelevant
test cases and added additional test coverage on
AdditionalSystemUpdatePreferenceController.

Bug: None
Test: RunSettingsRoboTests
Change-Id: I5224900bc3449a9f329a1d02ee3cb597b4748d91