[UWP] Don't re-use custom AdaptiveImage renderer for background images

[andrewleader]

Using the AdaptiveImage renderer to render background images causes strange things. As a host, if I override rendering of AdaptiveImage, I also override rendering of background images, which probably wasn't my intention.

At the working group meeting today (with Matt, David, Andrew, Bill, Alberto, Shalini, Becky, and Paul), consensus was that we shouldn't be doing this.

When a need arises for overriding background image rendering, we'll solve that with a separate API

[shalinijoshi19]

@almedina-ms assigning to you for load balancing; Also @RebeccaAnne / @andrewleader moving this out of 1.2 to make space for some of the other breaking/pressing bugs

[shalinijoshi19]

The work that this bug needs in XamlBuilder is being tracked by issue #2534

[shalinijoshi19]

@paulcam206 assigning to you; Do you just want to use this as a test for the rework you are doing as part of #2534 now?

This issue has been automatically marked as stale because it has not had any activity for 5 days.