Activate dark theme when the UiMode is UI_MODE_NIGHT_YES.

This change also activates night mode when the device enters
battery saver (saving substantial power on OLED screens).

Bug: 80307398
Test: adb shell service call uimode 4 i32 1 # nightfall
      adb shell service call uimode 4 i32 2 # morningtide

Change-Id: I9c08a749d4cd99ab7231363b045949eb2e830dfa