Hide Udfps icon when shade is expanded

1) Adds interface for ScrimController to notify listeners of new
   alpha
2) For SysUI modules such as UdfpsView that have views in layers
   ABOVE the notification shade (such as during lockscreen auth),
   yet need to pretend to be behind the shade (such as during
   enrollment), this allows UdfpsView to set its alpha and color
   to the right level

Bug: 177931181
Test: atest com.android.systemui.biometrics

Change-Id: I9b7bd5d72acee8ccbba9aef5b1c935bbf1da815f