Respect "power button instantly locks" setting for dream behavior.

Don't instantly lock the device on single power button click if the
"power button instantly locks" setting is off. This is only on devices
that have single power button press configured to start dreaming.

Bug: 263161209
Test: manually by turning "power button instantly locks" setting off,
enabling screen saver, docking device, and tapping the power button to
start dreaming. The device should not be locked. Then, set up again with
the same conditions, turn on the "power button instantly locks" setting,
then tap the power button to start dreaming. The device should be
locked.

Change-Id: I991504805c534eb76269579a2e753318fe0f1811