Add settings for scheduling dark theme

Adding a settings page similar to night light for Dark theme. it will allow the user to schedule
dark mode from sunrise to sunset

Test: make -j64 RunSettingsRoboTests ROBOTEST_FILTER="com.android.settings.display.DarkUIPreferenceControllerTest" && \
make -j64 RunSettingsRoboTests ROBOTEST_FILTER="com.android.settings.display.darkmode.DarkModeActivationPreferenceControllerTest" && \
make -j64 RunSettingsRoboTests ROBOTEST_FILTER="com.android.settings.display.darkmode.DarkModeObserverTest" && \
make -j64 RunSettingsRoboTests ROBOTEST_FILTER="com.android.settings.display.darkmode.DarkModeScheduleSelectorControllerTest"

Fixes: 141567787

Change-Id: I3094b655c04f59c23a95162a1be5ee8260e28a48