[flexiglass] Adds XML config to SceneContainerFlags

While removing the pre-aconfig SCENE_CONTAINER flag from
Flags.kt, I noticed that the original flag also checked an XML config
boolean which SceneContainerFlags wasn't checking. I've updated that
class to take the XML config into account, in addition to cleaning up
Flags.kt.

Bug: 283121968
Flag: NA
Test: Unit test updated
Change-Id: I6b44d1ae042ec5d3409675c3fb39b00f65b8c8c9