sysui: refactored for extension.

Refactored StackScrollAglorithm and NotificationStackScrollLayout to
implemment pinned top expandable view. For StackScrollAlgorithm, the
logic of computing Y translation and Z translation are extracted from
the for loop body.

Bug: 27101250
Change-Id: Id54af4d758e9ed692c01363de90e85dee3b2d026
(cherry picked from commit 0f5a0d2ebdb7a979af48b42a00ff8d1f7b57a5b7)