Launch DesktopWallpaper in fullscreen

This CL ensures that DesktopWallpaperActivity is launched in fullscreen
regardless of the TDA’s windowing mode. Otherwise, it’s launched in
freeform mode on freeform-first devices.

Bug: 363857269
Bug: 361419732
Flag: EXEMPT bug fix
Test: manual
Change-Id: Ibbd4b6d914cda5cb34d0edc56630713f56bb3d46