Rename SceneKey.Foo to Scenes.Foo

This CL changes the SysUI scenes accessors to Scenes.Foo instead of
SceneKey.Foo, making them normal `val` instead of objects. This will
make the removal of the fork of STL SceneKey easier.

Bug: 293899074
Test: m sysuig
Test: m SystemUITests
Test: Manual, enabled Flexiglass and used it.
Flag: N/A
Change-Id: Ib5233b53bc404e38217fe001cd98b03cc071cee3