[Dual Shade] Remove DualShade.isEnabled from FooterActionsViewModel.

This is needed to transition from flag-based activation of dual shade to
a setting.

Bug: 388793191
Test: Unit tests still pass.
Flag: com.android.systemui.scene_container
Flag: com.android.systemui.dual_shade
Change-Id: I893f452b4b8970d4f39a8c34985bacb24e2a2a6a