Don't skip setting scrim alpha when unlocked

The problem was that when there are different wallpapers on
lockscreen and home, the unlock transition doesn't complete until
the wallpaper has completed its cross fade. The notification alpha
was not being set until that transition completed, resulting in the
home wallpaper being visible if the user expanded the shade
immediately after unlocking. Checking whether the phone is unlocked
before skipping the update of the notification alpha fixes this
issue.

Fixes: 310394043
Test: manual
Flag: NONE
Change-Id: I0999163a3e3fc8a09d57243899d15ddd118febae