Update enabled state of ZenRulePreference

Test: manual
  1. Settings > Sound > Do Not Disturb > Schedules > Sleeping
  2. Toggle enable state of DND rule to ON
  3. Navigate back
  4. Observe: Sleeping rule is enabled visually (check box is checked)
Fixes: 140090071
Change-Id: I44078cdba9642eea93f9b6d2eb3eef486d3c38bc