[Light Switch] Hotfixes (#42434)

<!-- Enter a brief description/summary of your PR here. What does it
fix/what does it change/how was it tested (even manually, if necessary)?
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## Summary of the Pull Request
Adds new "Off" mode for the schedule mode options which disable the
schedule.
Adds explicit function to disable light switch by default.

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## PR Checklist

Closes: 
- #42402 
New behavior: "Off" mode added. when off, the regular service loop stops
and all actions are event driven to either resume the loop or listen for
hotkey.
- #42386
New behavior: Disabled explicitly by default
- #42389
New behavior: When switching from dark to light mode the system theme
will remove the accent color.
- #42513
New behavior: Manual mode no longer gets reset. It was being overridden
by the sun calculations that were invertedly running when in manual
mode.

Todo:
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx @alvinashcraft we will need to add this new mode
to the documentation.

## Validation Steps Performed
- Removed all default settings and tested new logic. Light Switch is set
to off by default.
- Updated UI and tested new "Off" mode, logs indicate mode switched and
ticker stopped. Polling resumes on mode change. (need to check that the
shortcut still works)

---------

Co-authored-by: Niels Laute <niels.laute@live.nl>
Co-authored-by: Gordon Lam <73506701+yeelam-gordon@users.noreply.github.com>

src/settings-ui/Settings.UI/ViewModels/LightSwitchViewModel.cs

@@ -39,6 +39,7 @@ namespace Microsoft.PowerToys.Settings.UI.ViewModels
 
             AvailableScheduleModes = new ObservableCollection<string>
             {
+                "Off",
                 "FixedHours",
                 "SunsetToSunrise",
             };