Use default val in night display repo for auto mode

When NightDisplayRepository reads automode setting, it used to
throw an exception if the setting was missing. Instead of that,
now the repo provides a default value, preventing the exception.

Flag: ACONFIG com.android.systemui.qs_new_tiles_future DEVELOPMENT
Fixes: 341125737
Test: atest NightDisplayRepositoryTest
Change-Id: I83f17182e8cd07df6438bf8e0f3115bc91790b1c