[Dual Shade] Expose ShadeMode only from ShadeModeInteractor.

This achieves two things:

1. Reduces the deps pulled in by callers who are currently injecting
   ShadeInteractor only to get access to the ShadeMode.

2. Consolidates all usages of ShadeMode so they're easier to track. In
   particular, this clearly marks all places to clean up once Dual Shade
   is launched.

BONUS: Exposure the convenience shortcut `kosmos.shadeMode`.

Bug: 388793191
Test: Unit tests still pass.
Flag: com.android.systemui.scene_container
Change-Id: I463ce9e2e0d459c073daccae5f0566a7546de2ab