[flexiglass] Moves transitions to SceneContainerConfig.

This should make it easier for variant owners and OEMs to provide their
own scene transition definitions, replacing our defaults.

Fix: 380358949
Test: built and ran with Flexiglass on
Flag: com.android.systemui.scene_container
Change-Id: Ieab374eba53c572294dd8bd3387306a6636aff22