Add methods to UserProfilePersonalizationSettings for retrieving lock screen and desktop background

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# Proposal: Add methods to UserProfilePersonalizationSettings for retrieving lock screen and desktop backgrounds
Today UserProfilePersonalizationSettings has methods to set the lock screen and desktop background asynchronously. I understand that getting the current wallpaper could violate privacy however some customization apps may need to access the lock screen/desktop background to do their job.

## Summary
To methods TryGetLockScreenImageAsync and TryGetWallpaperImageAsync could be added to Windows.System.UserProfile.UserProfilePersonalizationSettings. Since this is privacy sensitive, Windows could show a dialog box asking the user whether the user wants to allow the app to access the lock screen/desktop background once. Instead of a permission or a capability, this can be asked every time the app calls the API.

## Rationale

* More opportunity for developers writing apps that target the app container

## Scope

| Capability | Priority |
| :---------- | :------- |
| This proposal will allow developers to access desktop/lockscreen background while being inside a low IL process | Must |
| This proposal will allow end users to let apps access their desktop/lockscreen background | Should |
| This proposal will allow developers to access desktop/lockscreen background silently | Won't |

## Important Notes
Same dialog should be shown when the API is accessed from unpackaged apps although those kinds of apps can use old Win32 APIs to achieve the same which won't show the consent dialog.

## Open Questions

Start with the existing UserProfilePersonalizationSettings API and its asynchronous lock-screen and desktop-background setters. Define the consent behavior and the TryGetLockScreenImageAsync and TryGetWallpaperImageAsync contracts; done requires agreement on privacy, unpackaged-app behavior, and the proposed API surface.

Written by the indexing model from the issue text.