Improved the performance of the notification shelf

The shelf had a few inefficiencies that were adding up
when calculating the positions.

Test: runtest systemui-jank -c android.platform.systemui.tests.jank.SystemUiJankTests -m testNotificationListPull_manyNotifications
Bug: 32437839
Change-Id: Iac08a7c364a924f1d0c14258461383b431f0542b