[flexiglass] Replace the two footer vis flows with only one

This distinction is no longer necessary in flexiglass, so let's simplify
things.

I didn't want to duplicate a ton of tests, so instead I'm just
collecting from the correct flow depending on the state of
SceneContainerFlag.

Bug: 364028944
Test: manually verified that the QS bug that required this distinction
(b/329432313) doesn't happen in flexiglass
Flag: com.android.systemui.scene_container

Change-Id: I04a2e19f3fd27367ec3e375d510238e876c01778