Add master toggle to enable/disable screen savers

The new control toggles the Settings.Secure.SCREENSAVER_ENABLED setting,
which controls if a screen saver is enabled. Also removed the "Never"
option from the WhenToDream picker, since this is now controlled via the
toggle.

Test: locally on device
Bug: 222360260
Change-Id: Ibcf52b5e2d1ad0c295f32c4c18bfc89082d074ce