First bits of "add a mode"

* Preference below the modes list.
* Temporarily triggers addition of a mode with default name and type=SCHEDULE_TIME (type will be "manual only" later).
* Fixed sorting of modes in the list when refreshing (new modes were added at the bottom instead of where they should, the same would've happened for renamed modes).
* Minor polishes (extracted fragment launch to helper class, renamed item controller class for clarity).

Test: atest com.android.settings.notification.modes
Bug: 326442408
Fixes: 347198709
Flag: android.app.modes_ui
Change-Id: Ie276c92181c5374faf74592433595e7e15a5efc0