Redirect to new modes page when modes_ui is on.

The new modes page has essentially no content yet, but this is a rough proof of concept for the existing abstract classes and hopefully serves as a building block for future changes. It lists modes and those preferences lead to contentless pages that will be fleshed out later..

Flag: android.app.modes_ui
Bug: 327260745
Test: ZenModePreferenceControllerTest
Test: manual; individual pages only have skeleton functionality and should be unit tested in future CLs
Change-Id: I12f48b48f761e3c9ff1a173445b15f7536d34edb