ZenPreferences observe zen changes from USER_ALL

Zen manual rule changes are changed by USER_SYSTEM, so Zen Preference
Controllers must register their content observers for zen changes and
zen config changes to USER_ALL instead of just their own user id.

Change-Id: I75194b62ab5e5b21dffbf04bba5009659bb41d81
Fixes: 70028673
Test: make ROBOTEST_FILTER=ZenModeButtonPreferenceControllerTest RunSettingsRoboTests -j40