Fix Zen tests that were failing with modes_ui enabled

Actual fixes are in ZenModesListFragmentTest (was creating somewhat unrelated preferences with the actual ZenModesBackend, and isAvailable now returns true for them) and ZenModePreferenceControllerTest (no longer available with modes_ui). But also added EnableFlags(FLAG_MODES_UI) to all tests in notification/modes that lacked it, so we don't regress before we flip the flag.

Bug: 356334282
Test: atest com.android.settings.notification
Flag: android.app.modes_ui
Change-Id: I791b52cfb3fd9adea61079b4b41e456c8b3fe7bd