[flexiglass] Introduces forced disablement of Flexi by variants

This will be used by AAOS to turn things off.

Bug: 428975753
Test: built and ran System UI with Flexiglass on, on phone, from
sysui-studio without an issue; temporarily set
SceneContainerFlag.isEnabledOnVariant to false in SystemUIApplication's
constructor and proved that it cannot be turned on by turning on the aconfig flag.
Flag: com.android.systemui.scene_container

Change-Id: I24a714d300c26b9faa5c27bc1b867f4478dd30dd