Change zen mode schedules page rule handling

The approach of reloading all rules (and recreating all ZenRulePreferences) every time the rule set changed causes the switches on the page to stop working.

This change keeps the ZenRulePreference around as long as the rule itself is around and keeps it updated, while re-adding the preferences to the PreferenceCategory if needed due to rules changing.

Fixes: 229879326
Test: manual, ZenModeAutomaticRulesPreferenceControllerTest
Change-Id: I4eba41e8252cedd87ac866e4b97513970ca2d94a