Remove jank on DND schedule page

Only add/or remove preferences when absolutely needed.
Also fix a 'load data from backend' method that wasn't.

Fixes: 216747934
Test: ZenModeAutomaticRulesPreferenceControllerTest
Test: view schedules page
Test: add schedule
Test: remove schedule
Test: view schedule child page and return to schedules page
Change-Id: I237c2ca7ea89ee6e42354470a76712068a7f4dd7