[flexiglass] Bouncer scene adaptive layout improvements.

1. Better breakdown into "layout" and "area" functions.
2. Broke down the dependency on "StandardLayout" where all layouts
   eventually called "StandardLayout". Now each layout arranges its own
   "area" element functions directly.
3. Cleaned up foldability functionality, which is still used by both
   "standard" and "beside user switcher" layouts.
4. Better naming sceheme for layouts: "stacked" renamed to "below user
   switcher" and "side-by-side" renamed to "beside user switcher".

Bug: 300677757
Test: manually verified on 4 physical devices: phone, foldables, taller
foldable, and tablet.
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT

Change-Id: I4d21f7acf0acef64bad335ff2d045753a66ed7d0