Do not set wallpaper zoom on non-attached window

Make sure the window is attached before updating wallpaper zoom.
When the device boots, the first call to updateBlurCallback
happens before NotificationShadeWindowView is attached so it
doesn't have a proper windowToken.

Fixes: 165816006
Test: updated updateBlurCallback_invalidWindow
Change-Id: I99af7f5c27c637cf3365b7a622f73ffad91c1f70