Battery saver dark mode bug

When battery saver is on and the dark mode is in automatic
scheduling, the battery saver does not apply before the
screen turns off which is a bug.

some refactoring has been done also to speedup startup time
Test: atest UiModeManagerTest UiModeManagerServiceTest

Fixes: 148798405
Fixes: 149546098
Change-Id: I0bee6517b39216146681097262cf55c7192b0131
(cherry picked from commit 2a76f814)