Fix wallpaper screenshot

Wallpaper screenshot was broken since it would just screenshot the
entire screen. Updated wallpaper screenshot code to use the new
captureLayers API so the wallpaper layer can be specified for the
screenshot.

Change-Id: I594870583ddc2fb29c7eeafe003f20e4ee392a3a
Fixes: 69562019
Test: testWallpaperScreenshot