Mirror WallpaperWindowToken instead of WindowState

Mirroring a heirarchy does not preseve the root layer's transform
causing a flicker when switching wallpapers. Fix this by mirroring
at the WindowToken level preserving any scale and translation set
on the wallpaper.

Flag: EXEMPT bugfix
Fixes: 341830039
Test: switch wallpaper in homescreen
Change-Id: I4b81cbde1d486b1e2c952f6d59961d89314465a3