Allow dark mode to turn on/off automatically while dreaming.

Currently dark mode schedules will wait until the screen turns off prior
to switching between light/dark themes. This avoids interrupting the
user while they are using the device. However, on devices where
screensavers are prevalent, the screen may never turn off and therefore
we will never switch between modes. This change ensures that the switch
can occur while the device is dreaming.

Fixes: 306272618
Test: atest UiModeManagerServiceTest
Change-Id: Ide068813075fc410fc6c6b4531cb09b7d6e11a84