Fix TAPL OverviewTask to use correct resource to measure height

The background in DesktopTaskView wasn't added for accessibility, so it couldn't be retrieved from the view hierarchy during e2e tests. This view is necessary to calculate the correct size of the DesktopTaskView.

The changes in this cl:
- Add backgroundView to DesktopTaskView's children for accessibility
- Use the correct snapshot resource for desktop mode in OverviewTask

Bug: 353948500
Change-Id: I83746510849e4013d623d49cbaccebd81daba96d
Flag: EXEMPT bugfix
Test: TaplTestsOverviewDesktop