Fixes alpha issue in lock screen shortcuts.

This bug was introduced in ag/21077389 where we implemented "dimming" of
the views to support the remote rendering of the screen preview that's
shown in the wallpaper picker but never tested that it doesn't affect
the views when shown in the normal lock screen.

This CL fixes that.

Fix: 266875410
Test: manually verified that pulling the shade down or the bouncer up
properly fades out the lock screen shortcuts
Test: manually verified that the appropriate "dimming" of the
currently-not-selected button is working as expected in the wallpaper
picker

Change-Id: I9f3e6f7ec600567065d10e6f94a87096607817bf