Dark theme twilight mode backend and tile

When the user has Dark theme scheduling turned on (b/141567787), the quick settings tile subtext should reflect the following:
If Dark theme is on: "Until sunrise"
If Dark theme is off: "On at sunset"
If the user does not have Dark theme scheduled, the text should not appear.
This matches the appearance of the Night Light QS tile.
All starting windows snapshots will be deleted and splash screens
will be used instead.

Test: atest UiModeManagerServiceTest
Fix: 143874807

Change-Id: Ie2ce64b5c5544fffe76be7ec3f971eb7461f8c4d