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

tests/robotests/src/com/android/settings/dream/DreamPickerControllerTest.java

@@ -62,7 +62,8 @@ public class DreamPickerControllerTest {
     }
 
     private DreamPickerController buildController() {
-        final DreamPickerController controller = new DreamPickerController(mContext, mBackend);
+        final DreamPickerController controller = new DreamPickerController(mContext, "key",
+                mBackend);
         controller.displayPreference(mScreen);
         return controller;
     }