Remove flicker when unlocking + turning off screen

As soon as the screen starts turning off, powerManager.isInteractive()
becomes false. This forces the home wallpaper leash back to completely
invisible again, which creates a noticeable flicker.

Since the wallpaper is controlled through a leash, we don't have a
problem leaving it at some intermediate opacity. It will get cleaned
up and removed later.

Test: manual (unlock, power off during unlock animation)
Change-Id: I8e8cf659b1f72dd183825f3f0d27c12ddf3c59b2