[Dual Shade] Mark `ShadeMode.Split` as deprecated.

Once Dual Shade is enabled by default in SceneContainer, we'll remove
`Split` from the `ShadeModeInteractor`, effectively removing split shade
from scene container.

Bug: 376411622
Bug: 411365508
Test: N/A, only added annotation and a comment.
Flag: com.android.systemui.scene_container
Change-Id: I77178a96c365e5511ff779e06c2da50c76bd4bd2