SystemUI theme feature flag

Hide SystemUI theme under feature flag, this way we can enable it when
we're sure that all new strings were localized and the feature is ready
for prime time.

Bug: 63903361
Test: manual
Test: make RunSettingsRoboTests ROBOTEST_FILTER=SystemUiThemePreferenceControllerTest
Change-Id: I1093d414d951b71cab7cff10187c63bdd80eeee2