Don't set negative alpha values for the KeyguardBottomArea

Sometimes the NSSL#updateKeyguardBottomAreaAlpha() was calculating
negative alpha values. This resulted in the KeyguardIndicationAreaBinder
and the KeyguardBottomAreaViewBinder setting an invalid alpha value, and
toggleing the importantForAccessibility property on their views.

This CL removes these unnecessary updates, and hopefully helps with
reducing some jank.

Test: atest NotificationPanelViewControllerWithCoroutinesTest
Bug: 296578272
Change-Id: I339cbd2b4522a24345f430cb4f6a5f1a4579a02c