Don't observe Modes changes if MODES_UI is disabled

The ZenModesLinkPreferenceController is present in the Notifications and Sound screens, and although isAvailable() is false when MODES_UI is not enabled, it was still registering the listener for Settings changes, and calling updateState().

Thus, Settings would (likely) crash if Zen settings change while the user is in one of those screens, due to not finding the manual rule.

Fixes: 360498255
Test: atest ZenSettingsObserverTest
Flag: android.app.modes_ui
Change-Id: I7741809fd0028aace6ac58992be965701e64a2e3