Instead of unnecessarily creating them by hand. This also allows restricting the visibility of the ZenMode constructors (which should only be instantiated by ZenModesBackend in production code).
Bug: 360817586
Test: atest com.android.settings.notification.modes
Flag: android.app.modes_ui
Change-Id: Id5a232a5a0ffc7bc09652fc4b6e6dd2a6383a2ae