Use the prebuilt manual DND mode from TestModeBuilder in tests

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