Avoid lock screen flash when going to dream.

This is specifically when entering a dream from a single power button
tap on devices that are configured with that behavior.

Bug: 261662912
Test: manually by tapping power button to enter a dream and observing
that the device is locks at the same time without flashing the lock
screen.

Change-Id: I89ecce9cc39cb1cb6f7b19068908ba436643223c