[flexiglass] Scalable scene transition definitions in Flexiglass.

All inter-scene transitions are now defined in
`SceneContainerTransitions`.

The actual transition spec is defined in separate DSL files under
`com.android.systemui.scene.ui.composable.transitions`.

This CL also adds a default cross-fade transition for any scenes whose
transitions aren't defined yet.

Fix: 294911992
Test: Manually tested the different transitions on a device.
Change-Id: I0613830edd71fcd0bc140356cff54fc79348ac2d