[Night] Add a new night theme trigger: bedtime

This CL expands the existing UiModeManager APIs to allow users activate
dark mode at their configured bedtime schedule on supported devices,
i.e. devices with Digital Wellbeing preinstalled.

The CL added granular types for NIGHT_MODE_CUSTOM. There are two types
1. MODE_NIGHT_CUSTOM_TYPE_SCHEDULE
   This is the type for a schedule set up by users via the Settings
   app.
2. MODE_NIGHT_CUSTOM_TYPE_BEDTIME
   This is the type for a bedtime schedule set up by users via the
   Digital Wellbeing bedtime settings. Unlike
   MODE_NIGHT_CUSTOM_TYPE_SCHEDULE, Android framework doesn't have
   any information of the bedtime schedule. The activation of dark
   theme lives inside Digital Wellbeing

Test: unit tests: atest FrameworksUiServicesTests:UiModeManagerServiceTest
      CTS: atest CtsAppTestCases:UiModeManagerTest
      Manual testing via a system app integration
Bug: 210975231
Change-Id: I3b16e649a048f8a485b9e9d6464d4f251839320d