[flexiglass] Single source of truth for notifications horizon. position.

This determines the position of the notifications shade, lockscreen
content columns, HUNs, etc.

This is primarily a refactor with no desired behavior changes, except
that it fixes b/441943061 as well.

Fix: 441943061
Bug: 438557793
Bug: 441599011
Bug: 440416099
Bug: 442378170
Test: Manually tested by opening the notifications shade in all modes
 on both a narrow and wide screen form factor.
Test: Added unit tests and updated existing ones.
Flag: com.android.systemui.scene_container
Change-Id: I0478d84ebc1384757f70544b1ce1e1525f93e685