[Catalyst] Refactor DarkModeScreen

By virtue of KeyValueStore abstraction, the per-fragment state could be
moved into storage.

NO_IFTTT=Catalyst only

Bug: 375132235
Flag: com.android.settings.flags.catalyst_dark_ui_mode
Test: Manual
Change-Id: Ifff1c6e0b51cda981337b84008713c35a0a724ea
2 files changed