Update 'when to show' screen saver preference setting (1/2)

Add a new boolean config to control if we should show the setting
preference to set up when to start dreaming. Next CL to update the
availability status of the preference based on the new config.

Flag: EXEMPT config change
Bug: 417996476
Test: Manually no change in the current behavior.
Change-Id: I3b9c13e02c77ff5bb3998555c7037e5aa499e8e2