Fade out LS UDFPS affordance on QS expansion

Use the QS expansion amount to fade out the UDFPS
affordance instead of blinking out when qs starts
being expanded.

Also, don't immediately hide the lock icon affordance
when qs is expanded. Instead, we rely on the alpha
set by NotificationPanelViewController. It's the same
alpha used to fade out the keyguard bottom area.

Tidy up the UdfpsKeyguardView alpha updating.

Fixes: 228897571
Test: atest LockIconViewControllerTest UdfpsKeyguardViewControllerTest
Change-Id: I23e716261d2d8881fab3b4b420e2080ff5a4cb28