Improved the performance of the shelf / scrim

We are now caching the light status bar state and
are only reevaluating it if it changed / alpha changed.
Similarly, we now cache the background color for the
scrim, since that usually doesn't even change
in the shade, only on the lockscreen with transparency.

Change-Id: I417bf8286b8ccdfcfb143e8ea365b01c2b348be7
Test: runtest systemui-jank -c android.platform.systemui.tests.jank.SystemUiJankTests -m testNotificationListPull_manyNotifications
Bug: 33252359