[Dual Shade] Ensure !is ShadeMode.Dual is only emitted once.

It was emitted multiple times when switching between Single and Split
shades.

Fix: 436926610
Test: Manually tested by opening the notifications shade on a desktop
 device.
Flag: com.android.systemui.scene_container
Change-Id: I2f75a2202ffe715fdc25de5fabd2aed1205d60ef