Implements basic anchor-based scrolling.

This change introduces an alternate method of laying out notification
rows in StackScrollAlgorithm based on identifying an "anchor" view and
its position relative to the top of the container, then laying out the
rest of the rows outwards from there.  It also adds alternate logic to
everywhere NotificationStackScrollLayout reads or modifies the scroll
position to work with the anchor-based approach instead.  A binary flag
is introduced to toggle between the approaches.

In this change we implement the basic scrolling functionality only.
TODOs are added where functionality is missing, including:
- flinging/animated scrolling
- forced scroll/scrolling to a specific view
- clamping to window insets
- a11y scroll actions/scrolling state
- edge cases related to expanding/collapsing views above the anchor or
at the bottom of the shade

Test: atest SystemUITests, manual
Change-Id: Idd0711a7947733bd81c10feccda871b736c0578a